what dchan said, 2x.
once you have afew that fit well some further advice:
1)if you can get a removable liner that is washable do so. I am on my 4th helmet, mostly because after 100+ days they start to smell.
2) I would advise an over the ear model. i had a "shorty" but a branch glanced off the shell& dug into my head above my right ear. finishing a ski day is more fun than stitches. depending on how & where you ski of course.
3) pick a lighter colour, black helmets in the alpine sun alday make for a baked noggin.
again great choice.