A helmet should fit snugly enough so that when you shake your head from side to side, it doesn't move. Having said that, make sure there are no pressure points that are going to cause you discomfort down the road.
They should definitely be snug fitting. YOu'll know if it's too tight by the headache that hits about 15 minutes into the ride. Advice- wear it around the dealer showroom for a while and see if it's working for you before you buy one. Try on the different brands, too. Shoei fits me perfectly, but Arai and some of the other are unbearable. A snug helmet will also "break in" after some use.
The padding in a helmet will, over time, compact and make the helmet somewhat larger. I always notice that my helmet seems larger after I get a haircut. When it grows out again, it is all good.
Chickenhawk is right about trying several brands... We all don't have the same sized head, nor is it the same shape. Some helmets are more ovoid shaped than others, some seem to sit farther down the skull, some feel like a small sauce pan crammed on your head regardless of it's size.
I read somewhere; "If you have a $5 head, buy a $5 helmet."
~~~~~~~~~~~~~~~~~~~~~~~~~~~
"Four wheels move the body.
Two wheels move the soul!"
Mintbread wrote:The padding in a helmet will, over time, compact and make the helmet somewhat larger....
Good point, the girl I bought my bike from gave me the XS helmet she used and it fit me, but my new helmet is a M and it fits fine, too. The XS helmets padding is so broken in, that eventhough it fits my head, my lips almost touch the front of it.
Don't just assume sizes will be the same either. I have two HJC helmets, one XL and one L. The L is actually larger than my XL. Like some others said, get the helmet that fits, not the one that you hear is the best. Shoei for instance, does not fit me in the least bit. So there's no reason to spend $400 on a helmet that isn't fitting correctly, because a correct fit, is a safer helmet.
In Aus, it's compulsary to wear a helmet whilst riding
I cant add anything to what people have said about the fit, except that when I buy a helmet, I make sure it meets Australian Standards. Not too sure if the U.S. has the same kind of deal?