Here in Aus helmets are sized by centimetres. I'm anywhere from 56 to 60 depending on the shape, brand, and padding style.
My new helmet is a really nice, really pricey one (new show helmet, picked up for 1/3 of the price because it'll be 5 years past manufacture in Feb). It's a 59. My old helmet is a 58, and the helmet I've been showing in is a touch on the big side at 57. I borrowed a friend's 56 for one show and it fit perfect.
When my new helmet turns 5 it will be retired and destroyed, and replaced with a brand new version of the same... assuming I can find the $300 it will cost to buy a younger one!!