Here is some info on the head shape vs helmet manufacture...each helmet manufacturer make a certain "shape", like Shoei makes "intermediate oval" shape (which I believe is the most common head shape). Anyway read the article it explains it all and the helmet shapes by manufacturer are also on this link. Click on any helmet and it explains the sizing by shape as well as size.

https://www.revzilla.com/common-trea...et-fitment-101