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Helmets are like boots, if on a smaller scale - go to your local shop and try them on. Helmets are invaluably important, and thus you should probably have one that fits correctly.
Only after you've been to your shop and tried them on in person should you consider going back and buying it online. Remember, size doesn't tell everything, and every helmet has a different shape.
A few people have mentioned the Holt, but I am sorry, it is a cheap piece of plastic. If you just want something to protect you from your bare head smacking a rail, then yes, it is a good option. If you want a high quality helmet, don't get it. Put on a g10 mx, or smith variant brim and it feels worlds apart from a holt.
That being said, I think you can find a better deal on a Shiv Audio if you keep your eye out and don't need it right away.