Different companies size helmets differently.. A One industries large will differ from a Fox and so on and so forth. Its best to try them on locally, or go through a company that has a sizing chart for your helmet of choice... Sorry Im not much help.
I always wear a helmet thats one size bigger then I need because I put my hat on backwards underneath the helmet. Its something I started doing when I was young and just kept doing it because it felt comfortable
when i bought my blaster the guy gave me a helmet and gloves that fit me perfectly, even know the guy was like 6 foot and im about 5'5. he was really skinny.