Helmet question: Need reply asap please!

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.
 
and you want it to fit tight. there should not be any movement at all when you shake your head
 
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