If you're in the 5.5-6" range, you're average (bit on the big side, according to some research results). girth, generally is 4.85" as "average".
Many a guy's ruined his sex-life (and sexual organs) trying to get bigger. And, what for? Too big isn't going to fit in her comfortably. If you want to give her a stretched-out feeling... get toys for her. Get one of the extension sheaths/girth sheaths. It's a temporary measure (that DOES work). Frankly, don't invite her to the taping of "Fear Factor: Penis edition".
Big/huge penis = good for shock-factor... but, well, that only goes so far.