Make it out of cardboard to the size you think it should be. If it doesn't fit, increase or decrease your scale factor by 10 percent and make another one out of cardboard. Once you have the size right you can make it in whatever you like. Cardboard is very quick, easy, cheap and by the time you get the size right, you will be an expert on making helmets and the final one will be fantastic!
My advice having been there.