I think Flavor was talking about a balaclava
I'm certainly no expert on automotive helmets, as mine is a motorcycle helmet, but advice I can give: find a place that lets you try helmets on. Do not buy one online without having put it over your head. Many motorcycle gear shops will let you drop a helmet on your head and walk around for a while because they understand it's not as simple as picking one up and paying for it because it has a sweet flaming skull on the side. You need to spend some time in it, at least a half an hour to see if it's comfortable. I imagine you'd be able to do the same when buying an auto helmet.
Here's a few things to do:
-Take a headsock/balaclava with you since you'll probably be wearing one in-car anyways...and so you don't catch anything off of the demo helmets in the store that someone else has likely tried on before you...just in case.
- Start one size above what you think you'll need, better to start bigger than smaller. Put it on and see if you immediately feel a lot of pressure on your head...it's likely too small. If it feels really roomy and drops on with little effort then it's clearly too big. It should be a bit snug without feeling like it's crushing your head/face. If the size feels good overall but the cheekpads are making you make a duckface, you may be able to swap in cheekpads from a larger size helmet to give yourself a little more comfort there, as long as your face doesn't feel unsupported.
-Grab the helmet on the sides with your hands, or have someone else do it, and try to turn your head while keeping the helmet still. If you're able to move your head despite the cheekpads and other padding that's supposed to hold you in place, try the next size down. You don't want the helmet coming around on you in a collision or causing undue strain on your head/neck, and you want to be able to see, of course. Some movement will occur, but you shouldn't be able to twist it to where you're no longer looking through the visor and now looking at the inside walls of the helmet.
- Wear the helmet for about a half hour. You're going to be spending a good amount of time in that helmet, so you might as well see how it feels now. It may feel good for 5 minutes, but after 10-15 minutes it may start giving you a headache or the weight may be tiring your neck muscles. Shake your head around a little bit and see how much the helmet moves and how heavy it feels. Get a good feel for how it fits. Look around the rest of the store, shop around and let the time pass.
- Different model helmets are made for different shaped heads. Try a few different models from the same brand if the first one feels weird or uncomfortable. Some manufacturers, like Shoei and Arai, make a model meant for people with narrower heads/faces, and another meant for people with rounder/shorter heads/faces. The people in the store should know these things and should be able to size you up well for a helmet.
-Don't spend ungodly amounts of money, but don't cheap out too much either. When it comes to safety gear, be willing to spend the money to get a good product. Your life and health aren't worth a few hundred bucks' savings, right? For that matter, materials in helmets last about 5 years (provided you don't ever drop it, expose it to temp extremes, or put it to use and have it actually stop your head from getting caved in), so if it costs you $400, that's not a bad cost if it sees a lot of use over 5 years.
Again, I'm basing this on experience from riding, but I imagine it can't be very different for getting an auto racing helmet. I'd say it's probably even more important because you'll be in the immediate vicinity of many hard objects to make contact with. It's not really supposed to be a quick in-and-out process at the store unless you already known what you need, so set some time aside to go and try some helmets on for a part of the day.