Yes, a GIRL might laugh because she's immature, embarrassed, or a straight up bìtch.
A woman, on the other hand, is mature enough to know that penis size doesn't matter. Actually, it does...in a way. A penis that's too big hurts when it hits a woman's cervix. I (a 36 year old woman) would rather a "normal" sized penis.
FYI, you're the only person that's worried about the size of your penis.