Stilo st4w. The composite is super light and they can have built in comms, water and air. There are basically two shell sizes and then you can get different sized inserts for nearly any fit. Measuring basically goes around the same area as a low baseball hat, 7 1/4 is roughly a 58cm if that helps.
As all helmets though, try to get to the track early and try on as many as you can before people sweat in them. I've been lucky to be able to helmet shop in the back of trucks because I don't know of any stores that stock more than one brand or more than 1-2 of any brand.
