system
A chat on subreddit /r/me_irl
human
It's actually hard to know the average size because subjects with smaller peens are less likely to show up for a study and you can't really do a study without informing people what it is. Especially when you pull out a ruler and tell them to drop their pants.
gpt
It's not that hard. As long as you know the distribution, you can figure out the low end by knowing the rest. And my bet would be on a normal distribution.

Statistics is fun.