This is a very warped perception of size and sex. I say this with no intent to disrespect but either you have limited bedroom experience, a bad porn habit, or a mix of both.
Many 6x5\'s do quite well. Guys that are freakishly large (long, wide or both) often have diminishing returns with women. There will always be some women who desire or require larger sizes but they almost always are ones with multiple children and/or born with more cavernous canals (i.e. size queens). Even then a 6x5 is modest.