I'm going to call out an old fling about his dick size in front of his friends
Why would you call him out about his dick size? Would he do soemething like that to you?
Probably, that's why I have to call him out first.
Even if you feel that the fling might make the same rude comment about you, that is no reason to publicly say something rude first. It makes you look bad, and will make him feel free to insult you back. It would be much more mature to not say anything at all about your fling.
I don't care if they think I'm immature. I want to get them back for how bad they treated me.
I think the best way to get him back is to let people know how he treated you. Talking about his penis isn't going to make him look bad, just make people laugh for a minute.