Is the human fetus a person with the appropriate rights that follow or is the human fetus something less – a nonperson – and therefore not entitled to the inherent rights due a person? I argue for the former and against the latter to defend the position that the unborn human embryo is a full person at the moment of conception and should be afforded the full rights due human beings by their very essence.
SECOND THOUGHTS ON SIZE
If I had it to do over, I would have thought of a different title/angle for this section of my paper or perhaps not covered it at all, given that it is a weak argument only put forth by people who have no idea what they are talking about. On one hand, I thought it was important to write about because I have heard it (or some variation of it) so often but on the other hand; I am setting myself up to be accused of building up straw men for the sole sake of tearing them down. However, this is not the case, although it may be difficult for someone as erudite as Jim to understand how a person could hold to this view but I can vouch that I do hear this “argument” in a variety of settings.
I am not alone in this; prolific pro-life trainer and speaker Scott Klusendorf mentions this in a brand new book released just this year:
I speak at some of the finest Christian schools around the country. Yet without exception, I encounter pockets of students who think human beings can be killed because they’re the wrong size …. 
From the other end of things, a recent New York Times article featured a broadly similar argument (although the piece was on a much wider topic than just abortion):
In his State of the Union speech, President Bush went on to observe that "human life is a gift from our creator — and that gift should never be discarded, devalued or put up for sale." Putting aside the belief in a "creator," the vast majority of the world's population takes a similar stance on valuing human life. What is at issue, rather, is how we are to define "human life." Look around you. Look at your loved ones. Do you see a hunk of cells or do you see something else?
Most humans practice a kind of dualism, seeing a distinction between mind and body. We all automatically confer a higher order to a developed biological entity like a human brain. We do not see cells, simple or complex — we see people, human life. That thing in a petri dish is something else. It doesn't yet have the memories and loves and hopes that accumulate over the years. Until this is understood by our politicians, the gallant efforts of so many biomedical scientists, as good as they are, will remain only stopgap measures. 
*italics and bold fonts are mine, not the author's*
Things haven’t changed much; back in 1972, Dr. Bart Heffernan wrote an essay entitled “The Early Biography of Everyman,” in which he said this about these kinds of dead-in-the-water arguments for abortion:
“The blob theory, the main tenet of the tissue-of-the-mother school of embryology, has been advanced for public scrutiny without so much as a note of public criticism from the scientific community or the medical profession.” 
Maybe I should have called this section the argument from looks or the clump of cells argument or even the blob theory. At this point, I feel like Pontius Pilate: "what I have written I have written" (Jn 19.22). The lay version of this argument goes something like this, “How can something so tiny be a person? I mean, come on, it just looks like it’s a bunch of cells.”
SIZING UP THE SIZE ARGUMENT
One is immediately struck by the shallowness of such an argument: how does the way something looks on the surface determine its humanity or personhood? Pro-life advocate Steve Wagner coined the term sizism to describe this argument. He makes the point it is a form of discrimination to disqualify someone simply because their body is undeveloped. In the case of abortion, they are being disqualified for status as a person, which means they have no rights under the law, and can therefore be terminated at will.
The pro-abortion argument from size basically says those who are bigger have the prerogative to decide whether those who are smaller deserve to live or die. Is this fair? Abortion then is the meanest schoolyard bully of all time, killing the smaller kids because they get in the way of his fun on the playground at recess.
Even though size is never explicitly stated as a factor, there is an element of this train of thought in many of the more academic abortion rights discussions that focus on “competing rights” between the mother and her offspring: “I am bigger (i.e., more developed) than you so I can do what I want to you because you are an inconvenience to me.” In the course of his discussion of size and personhood, Wagner asks some great questions:
Why should we believe that microscopic human beings aren’t persons? How is your statement different from saying to a disabled person that he doesn’t count because of how his body looks and works? Should we call that developmentism, discrimination against someone for the body she’s developed? Think of a two-year-old: Isn’t she smaller than the rest of us? But she has equal value to adults in spite of her small stature. If she’s valuable, size is irrelevant to value, right? Isn’t the embryo valuable too? 
We should treat unborn persons – regardless of size – the same way we should treat any weaker or more vulnerable member of society; with care, dignity and respect. Minorities (as unborn persons are) in any society should be protected under the law by virtue of who they are, not because of their size.
Think about any argument made from the size of an individual implies: the bigger you are, the more of a person you are. Is Lebron James more of a person than Nancy Pelosi? Obviously, the answer is “no” and it sounds ridiculous to even pose the question – which is precisely the point. If an increase in size does not result in an increase in personhood, unborn microscopic humans are just as much of a person as you and I are; albeit, much smaller persons.
“SIZE MATTERS NOT – JUDGE ME BY MY SIZE, DO YOU?”
– Yoda to Luke while training him in the Dagobah swamp
A more penetrating question to ask is this: can any living being become anything else besides what it already is? How can something become a person unless its essence is already personhood? If the color blue is only blue and not the color red in the same way at the same time, its very essence – its fundamental property – must be blue and not red. Another example is that of the tadpole and frog. The tadpole is simply a name for a specific stage during a frog’s development. If one were to terminate a certain tadpole, then a certain frog would be terminated and no longer exist. This means you did not come from a fetus you once were a fetus.
Dr. Paul Ramsay expresses this same idea in different terms,
“Subsequent development cannot be described as becoming something he is not now. It can only be described as a process of achieving, a process of becoming the one he already is. Genetics teaches us that we were from the beginning what we essentially still are in every cell and in every generally human attribute and in every individual attribute.” 
This means while selves remain, stages pass.
Abortion advocates attempt to side step this issue by calling the unborn a "probability of a future person" or a "potential life". Tragically, even some evangelical philosophers have employed this confusing and misguided language. For a discussion of one such example - Norman L. Geisler - see Paul B. Fowler, Abortion: Toward an Evangelical Consensus.  Fortunately, Dr. Geisler switched his position on this issue in the mid-80’s; to compare his two positions, read the chapter on abortion in the first edition of his widely used Ethics book in 1971 and then compare it with the second edition. 
THE PROBLEM WITH THE PROBABILITY ARGUMENT
One way to think about the idea of probability (or potentiality) is that every adult was once an unborn person, just as every oak tree was once an acorn. An acorn is simply a mini-oak tree, just as a microscopic person is a mini-human.
The more gruesome – but true - response to this common dodge is citing a description of an abortion. Listen to a member of Planned Parenthood describing morcellation (morcellation is the technical term for when the baby is sliced into different sections during a D&E abortion) during a common late-term abortion procedure:
“The fetus was extracted in small pieces to minimize cervical trauma. The fetal head was often the most difficult object to crush and remove because of its size and contour. The operator kept track of the fetal skeleton.” 
This is not a potential skull being crushed or a potential skeleton being counted. These are actual – not merely “potential” – human body parts involved in this procedure. It is true the unborn are potential adults. The unborn are persons who are not yet adults but then again, so are teenagers!
In what should be a self-evident statement brought to my attention by Randy Alcorn, he points out, “One must have a head to be decapitated and body parts in order to be dismembered.”  Alcorn points out this obvious fact as a commentary on Dr. Warren Hern’s abortion manual, where the abortionist gives these instructions, “A long curved Mayo scissor may be necessary to decapitate and dismember the fetus.”  Alcorn then says what should be self-evident: “Human body parts are the product of actual human lives that have ended”. 
Noted author James T. Burtchaell puts it succinctly when he writes, “What we honor in humans is not their stages but their selves. The unborn is not a potential human but a human with potential.” 
Prenatal surgery – as well as the photography involved – has shed more light in this area as of late. “We don’t treat the fetus as a potential person,” says nenonatal specialist Dr. Thomas Elkins, but instead
We have been approaching the fetus as a patient for a long time … We can do a lot of things for that fetus that basically elevate it in every way and every sense to personhood. We operate for the benefit of the fetus … we monitor the fetus … We intervene when it appears ill and rush in to save its life. And I mean rush. It’s a two minute dash to the C-section room to get out a fetus who has collapsed its cord. It’s a dash for a life … 
SIZE, PARTS, & MACHINERY
There is a certain component of the size argument built into to the “parts” argument: this is the idea that since the unborn does not have a certain part manifest yet, it is not a person. Some people mention the first sign of brain waves; some - who are decidedly less informed - even cite the structure (or lack thereof) of the face.
John Walker, a writer for the group Libertarians for Life, has a helpful discussion about the idea of “machinery” and then supplies an appropriate response:
The question is how reason and choice make us persons. Is it a matter of a power we have? (Or a potential, capacity, etc., etc.) Or is it a matter of an act in which we engage? (Or a behavior, manifestation, etc., etc.) …
If act is what counts, how can we include in the category of person those who act at a lower level than others we exclude? …
I think the response [of the abortion advocate] would be that at some point "the machinery's in place" – the cerebral cortex, etc. But if the machinery isn't producing the output, then we are talking power again, not act. And talking power, one has to face the fact that the power exists before the point at which the cerebral cortex (or what not) is produced. "The machinery's in place" in the zygote, too – after all, the nature of this "machinery" is to grow. And even at birth, the machinery still has growing to do before the human infant will manifest any superiority to a number of the lower animals. 
Pro-lifers admittedly went too far when they attempted to turn Dr. Seuss into a pro-lifer by co-opting the Horton Hears a Who? movie because of the refrain “a person is a person, no matter how small” but what about Shakespeare? No one has exploited him yet, have they? Let me proceed to do that then, before it comes passe:
Hath not a Jew hands, organs, dimensions, senses … ? If you prick us, do we not bleed? … If you poison us, do we not die?
– Shylock in The Merchant of Venice
UPDATE: Added the context surrounding the Michael Gazzaniga NY Times quote and changed the wording that leads into the new fuller quote(12/16/09)
 Scott Klusendorf, The Case for Life (Wheaton, Ill: Crossway Books, 2009), 455.
 Michael Gazzaniga, “All Clones Are Not the Same,” New York Times, February 16, 2006. http://www.nytimes.com/2006/02/16/opinion/16gazzaniga.html?_r=1
 Abortion and Social Justice, edited by Thomas W. Hilgers and Dennis J. Horan (New York: Sheed & Ward, 1972), 3.
 Steve Wagner, “The SLED Test – Four Top Arguments,” http://www.str.org/site/DocServer/2.1_four_top_arguments.pdf?docID=861
 Paul Ramsey, “Points in Deciding About Abortion,” The Morality of Abortion: Legal and Historical Perspectives, ed. John T. Noonan (Cambridge, Mass.: Harvard University Press, 1970), 66-67.
 Paul B. Fowler, Abortion: Toward an Evangelical Consensus (Portland, Oreg.: Multnomah Press, 1987), 76-77.
 Norman L. Geisler and W. Watkins, Ethics: Options and Issues (Grand Rapids, Mich.: Baker Books, 1989).
 Sadja Goldsmith, et al., “Second Trimester Abortion by Dilation and Extraction (D&E): Surgical Techniques and Psychological Reactions.” Paper presented at the annual meeting of the Association of Planned Parenthood Physicians in Atlanta, GA, 13-14 October 1977, 3.
 Randy Alcorn, Pro Life Answers to Pro Choice Arguments (Portland, Oreg.: Multnomah, 1994), 54.
 Warren Hern, “Operative Procedures and Technique,” Abortion Practice (Boulder, Colo.: Alpenglo Graphics, 1990), 154.
 Randy Alcorn, Pro Life Answers, 54.
 James Tunstead Burtchaell, Rachel Weeping: The Case Against Abortion (San Francisco, Cali.: Harper & Row Publishers, 1982), 85. Dr. Denis Cavanagh said almost the same exact thing in his inaugural address as professor of obstetrics and gynecology in 1972 at the University of Tasmania. Cited in F. LaGard Smith, When Choice Becomes God, Eugene, Oreg: Harvest House Publishers, 1990, p 126.
 Harold Smith, “A Legacy of Life,” Christianity Today, January 18, 1985, p 18.
 John Walker, “Power and Act: Notes Towards Engaging in a Discussion of One of the Underlying Questions in the Abortion Debate,” 2000. http://www.l4l.org/library/poweract.html.