One that is to loose will bounce around while you ride, and impair you vision.
One that is too tight will pinch your ears, limit your hearing, and cause you to heat up since the air can't flow correctly.
If you are involved in a crash, make sure to check the helmet thoroughly, since any cracks or hard blows will compromise it's integrity, and leave you with a false sense of security when you ride.
With that said, check out this selection of motocross helmets, they all meet ANSI, SNELL, and ECE requirements. So whether you are buying for simple protection, or even to make a bit of a fashion statement, ATV-411 has got your helmet needs covered.
Ride Safe. Ride Legal. Ride Again...Curtis