Or we could skip that weird shit and just measure the damn thing. If you're above 5.5, you're above average. If you're below, well, that just means sex feels less awesome for whomever you're fucking. S'long as you can bust a nut, I don't know what you're worried about. Learn to go down, you're all good.
Also, if you can comfortably take a dick as long as a #2 pencil that won't fit in a roll of toilet paper without crying, you might want to get a vagina tuck.