Before you buy a small you should consider the amount of hair on her head. Plus most woman wear some type of scarf to protect themselves from helmet head after riding. I have never had a girl fit into a small helmet unless she is a bald headed and I only ride petite women. You may want to go with a medium.
That's an interesting question... you might as well ask what the average shoe size is. It will depend on the the brand of the helmet (they are not all sized the same) and the amount of hair the female has.