When doing research for a helmet, get some idea of what kind of head shape you have (long oval, round, egg, etc) Most helmets are "neutral", meaning they will fit "most" heads, but putting a long oval head in a neutral helmet won't work out too well. Probably 80-90% of people have a head that will fit fine into a neutral shaped helmet, but you'll have "hot spots" if not. Some of that can be "adjusted" with a bit of modification the to foam inside the helmet. Arai is one of the very few manufacturers that actually makes helmet lines for different head shapes, but the types of helmets they offer are limited (no Flip Face). Also, see if the manufacturer uses two, or three, shell sizes. Many will use only two shells...one for S, M, L and another for XL, XXL, or some variation thereof. Some of the ones who have three shells will use one for XS, S, M, another for L, and another for XL, XXL. If you've got a small head in a large shell, you might look like a goober...
Hate to sound like a broken record, but go to Webbikeworld. They have an in depth discussion on head shapes and how to properly fit a helmet. They also include head shape criteria in their reviews, and their reviews are VERY thorough. Revzilla also does some nice Video reviews of various helmets. Those two sources about cover everything you want/need to know about helmets. Revzilla is an excellent place to buy a helmet....no return fees and they will bend over backwards to make sure everything is right. Their prices are generally very good too.