I have my own system that takes length and girth in combination. Multiply your length by your girth and your result is in square inches. Surface area. It's approximate, but close. Big head? Add 1 square inch for every 1/4 inch your head is bigger than at midshaft.
Itty Bitty Anything under 10. Same as micro.
Little 10 to 20 square inches.
Small 20 to 30. This is my category.
Average 30 to 35.
Big 35 to 40.
Too big 40 and above.
I've been with some in all categories. The little ones are easy on the jaw muscles. The huge are fun to play with but that's about all.