helmets may not be cool but i always wear mine...
anyways...Pro-tec seems to be the best company for bmx helmets that arent full face.
like mude said, try on a freinds or go to a bike shop to buy one.
also if your going to get a pro-tec try to get the one with all the soft foam not the hard foam and the 2 cm thick comfy stuff...becuase those fall off...and that would defeat the purpose of a helmet.
good luck
try
www.albes.com
they have the good helmets...(once you find you size of course)