I usually categorize beetle sizes as - XS, S, M, L, XL, Monster.
Since this species is known for their huge head, id say if they have a small head, then its a minor.
Small head meaning the width of the head is about the same as the rest of the body.
They will have a higher chance of having a big head above 47mm.
Anyways, I would say...
30-35mm- XS
35-45mm- S
45-50mm- M
50-55mm- L
55-60mm- XL
60mm+ - Monster