If you are not able to make it to a tack shop, and must order online, I have found that in general, Troxel has a shape that fits a rounder head, whereas Tipperary fits a more elongated head. Most of the helmets now have tensioning dials in the back , so very adjustable. I only mention Troxel and Tippearary because those are the only two I have any experience with . They are both fair in quality and price, though I MUCH prefere Tipperary.
I have had mine for about 4 years, had 4 falls, but none of them entailed having my head smack the ground. I always seem to land on my tush or my back.