This is for Shoei, Arai, and HJC helmets. It should give you an idea.
Fitting: Put the helmet on by grasping the chin strap in each hand and pulling it on over your ears until you feel the top portion of the helmet touch. If the helmet immediately feels too tight, ie; your head does not touch the top of the helmet, you cannot pull the helmet down over your ears, or is painful, try the next size larger.
Once you have found a helmet that feels comfortable and snug, you must assure that it is not too loose. To do so, with the chin strap securely fastened (see chin strap diagram) grasp the helmet chin bar firmly with one hand and try to move the helmet from side to side and up and down. If the helmet slides on your skin and hair, it is too large. If your scalp and skin want to move with the helmet, the fit is snug.
Finally, while still grasping the chin bar, try to rotate the helmet as far down and forward as possible then as far backward as possible. If the helmet feels as if it might slide off in either direction, the helmet is either too big or the chin strap was not securely fastened. Additional note; HJC has a wide range of helmet sizes, but if for whatever reason you cannot find a HJC helmet to fit your head size or shape securely, keep looking.
Each helmet manufacturer is a little different. The link below has sizing based on brand. Hope this helps.
Link:
http://www.motorhelmets.com/htm-service/sizingchart-helmet.htm~Brian
