It might as well be said that a general rule when it comes down to insults is that people insult others about something they are familiar with.
You see, people do like to have understanding of the subjects they talk about, and it is entirely natural that when people insult others they do so from a position of understanding.
As many will know, I am what you'd call a psychologist - but primarily deal with words and their usage. The one-off usage of an insult I am quite able to overlook, but it has been several times now that I have seen you trying to degrade people you don't know with concerns to their penises. As a professional, I can tell you why you do that, but it's clear you wont like the answer - even though you will be well aware of the answer.
You and I know the answer and that's fine, I never considered human brilliance, intelligence or anything else of that nature as having to come down to the size of the sex tool. To me it is largely inconsequential as long as you manage to win the battle and impregnate the woman - in which case it is clearly large enough, but to you it seems to be a sticking point, and I can only hope you get the appropriate help to get you over that hurdle.
Anyway, the point is that you're giving away more information about yourself than you really need to or should, and as such I would state that if you're going to insult people, (which is your right), you choose something that isn't going to reflect so negatively on yourself, (to the trained eye).
Enjoy your day.