This anatomical Holy Grail is the source of much strife, immaturity and insecurity amongst younger males with no conception of the inner workings of its female counterpart. In actuality, rigidity is a far more important factor in stimulating the female genitalia than either girth or length. Girth, secondarily, is often said to be noticeably more important than length, provided one possesses at least the length of the girl's longest finger (tiny girls don't require the same proportions that tall girls do).
Five and a half inches is the average length of the American penis, so chances are very good that you have all the equipment you need. Relax, watch some porn, learn how to use what you have.
Sam: "How the hell does Ying get all the girls? He can't have more than four inches!"
Joe: "Cocky bastard knows how it's done, I guess. Wish he'd teach us something... No homo!"
Sam: "Haha, 'no homo' indeed! I still say you need a big penis."
Joe: "Aaaaand that's why you're still a virgin."