I can honestly say I've never encountered anyone I've felt was too small. That's not to say that someone couldn't be "too small," but I'm going to say 3" or less hard.
That said, girth is important. I'd rather have a 5" thick penis than an 8" skinny one.
6" is average, i can't understand why some females say don't contact them unless you are well endowed, cos size doesn't matter, and i will always stand by that, recon I am about average 10" only kidding..lol...