mine is sort of embarrassing, i didn't find out until i as 18. I was with the same girl from when I was 15-18. We had sex the entire time, but only with each other (we lost our virginity to one another), so neither of us knew any different.
we would watch porn, but compared to the porn stars I'm about the same size (9x7) so that just confirmed my belief that i was average. Anyways, one day she asked her best friend how on pornos the girls 'always take the whole penis inside them', the best friend then quickly spread a rumor that i must be pretty long since she asked such a stupid question.
Anyways, we broke up, and i kind of went on a fucking streak. I think i slept with about 6 different girls in 2 months, and every single one made a comment. So that is when I first learnt.
My idea was cemented though, when about 2 years after breaking up with the first girl, i saw her drunk in a bar and she told me "the next guy i dated after you, i was so disappointed when i saw his dick. I had no idea you were so big"