I am a 16 year old guy (though I've never had sex) and I just have to say that small breasts honestly turn me on. Medium sized breasts (B's and into C's i guess?) are also attractive but not much past that for me. Even girls that are just barely flat give me erections in school (which, for all you flat girls listening, seems to me more embarrassing than having small breasts). I can just admire the rest of some girls' bodies.
a big turn off for me though are the girls that wear heavy sweatshirts because they think their breasts are small. Really, girls, wear some spaghetti straps and let it all show! I promise, guys WILL look at you no matter what!! (at least I do...)