just measure your head and if your planning on wearing a beanie under it get one size up unless you barely fit that size. Most helmet companies have different sizing so check each companies charts.
Go into a shop and try some on. Give each one a long trial on your head. You want a helmet which fits you so well that is feels like a hat, and does not bother you at all. I would not order one you have not tried on first.
i would recomend the bern baker, its light, stylish, and you can get it for around 60 bucks at some sites, another helmet i recently found out about was the "Leedom Boss". it was the helmet that jossi wells wore at the x-games a few years back when he got silver in slopestyle and its a really steezy helmet with a low profle and a great look, plus you can get it really cheap for about $45 in L or XL at this site- http://www.snowshack.com/detail/SNW+LD-02144+CW+2XL_Leedom+LDM+Boss+Ski%2FSnowboard+Helmet