The Size Question
My guess is that such preferences are highly individual and based on the woman's own anatomy. Presumably, with the right moves, a smaller penis is just as capable of satisfying most women as a larger one, but there's likely to be some need for physical compatibility. The G-spot is generally assumed to be located between 1 and 3 inches deep on the front wall of the vagina, so even a very short member is capable of reaching it, and the highest density of nerves in the vagina is near the entrance, so length isn't necessary to provide a lot of stimulation. The cervix also has a lot of nerves, and some women enjoy having their cervix bumped, but others find it painful, making a long penis a bonus for some and a problem for others. Not to mention that for a lot of women what you do to the clitoris is more important than what's going on in the vagina. Vaginas vary dramatically in size as well and some penises may just fit better than others. If you happen to be small a larger penis can be physically uncomfortable while a smaller one may get the job done without pain. Conversely, a woman with a larger vagina may not get as much stimulation as she prefers from a smaller penis. (I didn't find any data on this, but I assume gay bottoms also have personal preferences based on anatomy rather than an arbitrary preference for big penises.)
The average vagina ranges from 2.75-5.75 inches (6.95-14.6 cm) in length and 0.8-1.4 inches (2.1-3.5 cm) in circumference at the entrance when not aroused, but has the ability to expand when aroused (or during childbirth, obviously). Not surprisingly, the size of a woman's vagina is roughly proportional to her overall size -- petite women tend to have small vaginas while taller women tend to have larger ones (I assume penis size also correlates with overall build -- that's certainly been my personal observation). Having given birth and age generally correlate with increases in size of the vagina, but unlike men and penis length, women have some control over such things, since the vagina is basically a muscle and can be exercised to increase tightness. However, being too "tight" is at least as common a worry among women as being too "loose." (I really hate using the terms "loose" and "tight" in reference to vaginas, by the way, since they're based on the presumption that one is better than the other.) While in theory the vagina should be able to stretch enough to fit a range of penis sizes, some women have trouble relaxing enough during sex, leading to discomfort and pain (lack of arousal beforehand can contribute to this problem -- it's not just lubrication but also the loosening of the vaginal muscles that makes penetration comfortable). In rare cases, the muscles involuntarily clench so tightly that penetration is impossible (a condition called vaginismus).
There's some evidence that while physiologically most women don't require a huge cock for sexual pleasure, some prefer one for psychological reasons. Our culture's "bigger is better" mentality may lead some women to choose to skip sex with less well-endowed men in favor of men who fit their stereotypical definition of masculine (i.e. big). Those women are idiots. That said, some penis sizes will work for some women and others won't, and the same can probably be said of vaginas and men. If a partner isn't satisfied with what you have, try thinking of it not as a personal inadequacy but a compatibility issue - like trying to fit the wrong puzzle pieces together. It's not that either piece has anything wrong with it; they just don't happen to work together.
Dr. Pisaster has a doctorate in biophysics, not actually anything sexy. She does however enjoy having sex, reading about sex, and talking about sex. Especially when she's had a little whiskey.