There is, in fact, a correlation. At least if certain studies are to be believed. The thing is, the correlation is small, and even with a positive correlation between height and penis size that absolutely does not mean that every tall man has a big dick and every short man has a small one. Just that, on average, if you take 1000 tall guys and 1000 short guys, the group of tall guys would have a larger mean penis size. But not by a lot. Your 6'7" friend might be hung, and he might not be.