Your Head. The metric way to measure is simply the circumference of the ole' noggin (not some arcane sizing like 7-3/8). If you have a tape measure, just run it around at about the point where the brim of a hat (or the helmet) would run. 1 inch = 2.54 centimeters.

But, I would absolutely concur that this is a purchase to be made through trying on the helmets. I just bought new helmet myself. I found only one or two helmets from about 8-10 models which really fit me properly. Most of the others were either more round or more elliptical than my head.

Also, pay close attention to "hot spots" or pressure points when you try the helmet on. If you notice a pressure point when trying it on, recognize that it is likely to blossom into a headache after 1/2 hour on the bike. When you try them on, try to leave them on your head at least a minute or two to see whether something develops. I rejected at least one attractive model due to an obvious pressure point.

Best of luck,
Jamie