question about helmets

I'm looking into buying a helmet. All I do is trail ride. Since the stores in my area don't carry them I was looking online. Any suggestions for a good helmet for trail riding? Also how do you figure out the sizing. Not sure of my hat size. Don't want to spend the money to get it and it is too big or small.
Thanks for your imput.

I have the:
Ovation Deluxe Schooler helmet. And I really like it fits my head shape well, light, nicely ventelated.

Personally I would say even if you have to drive a bit find a store that carries a few different brands. I tried on some troxels and they seemed too round (basketball head) the ovation and the tipperary sportage both seemed to be more oval (fit my head much better) the only thing I didn't like about the Tipperary Sportage was the piece that comes down behind the head, saleslady said some love it some hate it. Just didn't fit the contour of my head at all.

It's easy enough to measure your head with a measuring tape. All the helmets for sale have a sizing chart based on the circumference of your head, so there's no reason to guess.

Helmets fit differently depending on their shape, so you might take a large in one style, while you'd take a medium in another.

It's definitely worth going a bit of a distance to a store to have one fitted for you.
Then maybe you could get it online for cheaper? ;) I worry about buying helmets online though, you don't know if they're roughly handled by the postman!

I wear the Tipperary Sportage. It is the only one I have ever worn so I can't speak for the other brands. Mine is a medium and fits me well and is comfortable.

In my experience. Troxels will fit round heads well, but if you do not have a super round head, it will not fit.

Since I don't have a round head, I have instead gone with Tipperaries, or Ovation Schoolers (my current helmet is an Ovation Deluxe Schooler).

I have a tipperary sportage for schooling and I love it. It's supposed to be one of the few helmets that protects the base of your brain stem or something.... but it's surprisly lightweight, feels very sturdy (sturdier then a lot of the cheaper troxels, IRHs, etc) and when the wind blows I can actually feel it go through my helmet. Plus its only like $60.