If you're trying to find a helmet, the best thing to do is go to a bike store with a friend and a mirror and try all of them on. My GF just bought a Giro that's pretty small and fits her head well. It was also one of the cheapest ones. It seemed like each model was made not only for a certain riding style and price range but also for a certain head size and shape.

And there's no reason to pay $200 to cover your head in styrofoam.