Utilize this Anatomically Accurate Human Head model to properly size your helmets, hats, headwear, etc. in your editing software to ensure the perfect fitting print! (I prefer Blender3D).
 

The "Base.stl" version has the nose and ears sliced off so that you can measure your head with simple outside calipers, then enter those measurements to precisely size the head to your desired size.
 

If you need the ears and nose for visualization purposes, the "Grouped_Obj" is an .obj file with the "base" version, with the ears and nose as a collection of objects. You can still resize the main section as you do with the "base" stl.
 

Quick and easy. So much better than trying to use the measurement tools in your slicer or editing software.
 

Cheers.