Shouldn’t you be starting grades at a “B”? An above-average dick warrants an above-average grade, does it not?
You also have to keep in mind the person giving out the grades (i.e. the receiving vagina.)
For example, some vaginae will grade anything over 6” long as hitting her cervix and a “D” because ouch!
But I think all vaginae will grade a “nice girth” as anything over 5. All vaginae can open sideways enough to squeeze out baby heads. I would agree that 5.5” girth will be uniformly called big. 6” of girth is the whooper.