I wear an XL size helmet. It's by Pro-Tec and they're mainly designed for skateboarding, but are also sort of ment for biking. They're big helmets though, and make your head look huge. The big advantage over a normal bike helmet, is supposedly with the pro-tec helmets it can take a few hits before it needs to be replaced (because skaters land on their heads alot?).

Yesterday, when I was carrying it in the check out line at the grocery store, the clerk asked me if all the scrapes were from accidents. I said "yes." though so far it's mainly only protected me from low hanging branches.