There IS such thing as too small, and there IS such thing as too big...but, if you think the size of ones penis automatically means "good sex" then I automatically think you're probably horrible in bed.
In most any situation, not just sex, the specs of a tool mean very little if you don't know how to use it.... if you don't know what you're doing you can have an decent tool and do far better work with it than someone who has the ideal tool, but has little to no idea of how to make it function.
I think, when it comes to sex, most men don't grasp this concept... I also think most men aren't very good in bed. Most certainly this is no coincidence.