Hello all, I am a beginner at doing this kind of thing but so far I have enjoyed greatly. I downloaded the Halo 5 Achilles helmet and eagerly just printed it off without sizing, at the time I did not know. Well at the end of the build I came out with a very small helmet that fits my fist. So I adjusted the size of the helmet only to make it far too big. I am not sure what the proper way to measure my head is to insure a good fit and proportionate size to the rest of my body.