I got the XS for my tiny head and it fits great. I wouldn't wear one of those spaceship helmets and this was the only one i found that doesn't look ginormous on top of my head. The strap in the back doesn't do much but it tuns out I don't really need it. Doesn't fit as well without the visor, but it seems to ventilate well anyways. And I can still fit a thin hat underneath when its really cold.