Science of penis size
I found this nice site that includes results from different studies on penis size.
5 size groups
This page uses 5 groups, some other add an XXL group and use 6.
Average numbers are often skewed by the larger numbers (average of 2, 4 and 100 is 53 but that number is meaningless really) it might be better to use percentile groups such as the median (50% is bigger, 50% below) or 90% where 1 out 10 men is bigger and 9 out of 10 are smaller.
It can be slight concluded from the 5 groups. You can see for example 75% is below 16cm. So I guess this group is for the other 25% please post your penis stats and pictures :)
If anyone has more studies please post.