Aaagh. The only time the size of a penis matters is in the realm of fantasy -- where it stands for what the little boy meant when he told his mother, while she was putting jam and butter on a slice of bread, "Please Mum, give me too much."
Otherwise, in the flesh and blood world, small penises are far preferrable: 1) because they don't hurt 2) because their posessors don't assume that the size is "enough" therefore they pay attention to other things like foreplay, motion, and rhythm 3) because the female vagina is far more sensitive at the entry point than deep inside...and extremely elastic. Let's not forget that the vagina finds a tampon (1/2" diameter, 3" length) an adequate fit.
Joanna