Hi dmar,
She's not very mature for 16.
You sir are NORMAL. Average for an adult, erect, is 5-5 1/2". Flaccid size varies considerably. Obviously she doesn't understand that. You are most definitely a "grower."
Don't be depressed. You have nothing to be ashamed of.
Talk to her. Ask her what was funny and then answer her questions as best you can.
Good luck.
You need to remember that you and her are teenagers. That was not a polite response that she gave to you nor a confidence boost to your ego - reverse this to her, how would she like it if she took her top off and you laughed at her breasts? Exactly my thoughts.
You are absolutely an average size actually for a grown male which you have growing left to do so who knows what size you will end up being and last laugh might end up on her after all (maybe you'll be hung? never know). Growth hormones do not slow down until you are about 20 or so.
If she really likes you she'd apologize about her bad manners but she hasn't. So don't get too hung up on her and get all depressed about it cuz as I said you are normal already.
Hope this helps