I havn't exactly fitted a bunch of helmets recently, I've had the same once since the 8th grade and I've graduated for college... but the idea is that it is comfortable but snug. Saying that, alot of ppl like to wear beanies and such underneath the helmet, so you gotta consider that... Mine seems more than warm enough by itself and its a pretty shitty one. The timeless test you always see ppl using seems to be the ol' put it on, snap it, adjust strap, then shake the head quit a bit in all directions, the helmet will obviously move a bit, but it should always stay in position.
my 2 cents..