I've never understood how a woman can stoop so low as to insult a man about something that most men are insecure about. I can only recall making fun of a guy's penis once, but that was not to his face - he was a guy I slept with and then he rejected me, and to make myself feel better (deliberately being immature to make myself be able to laugh at the situation even though I felt crap about being rejected in such an inconsiderate manner) I used to make up songs with my flatmates about how small his penis was. But I would never say that to a guy's face, and I'd never make fun of a guy's penis if I had even an inch of respect for him, no matter how badly the relationship ended. And to be honest, even though it was a bit small, it wasn't TOO small.
Anyway. On to penis size. Seriously, men make it a bigger deal than women do, and that's why women know exactly where to hit them where it hurts. A guy I was with recently had a penis which I thought was the perfect size for me - around 6 inches and quite thick - and he had a complex because an ex had made a comment of the same sort. To be honest, I've been with a fair number of men (around 30), and I've only slept with one guy who was too small for me, about four inches. He was also not very giving in bed anyway (hated oral, wasn't too good at fingering), and if he had been better, then I probably wouldn't have minded that his cock was small.
I hope this makes you feel a bit better. Penis size is NOT as big a deal as most men make it out to be. I'm not going to lie and say I wouldn't prefer a nice thick 6-7-inch to a 5-inch, but I'd rather have a good lover with the latter than a mediocre lover with the former. As long as you don't have a micropenis anyway....