I think you'll find that even a medium in a Shoei, Arai, HJC, Icon, or Shark all fit differently. My head shape wasn't read for the Arai's, they just didnt fit well. I went with a Shoei RF-1000, in a medium, it fits perfect. Was tight when it was new, but once the padding 'wore in' a little, its just right now. The last thing you want is a helmet that is too loose. As for the Snell standards, you'll want the 2005, thats the new standard.