Make sure to choose one that covers your whole neck and face. Ideally, a ski mask will cover the entire face and neck, but you may want more coverage on the front than you do in the back. Some ski masks have adjustable straps, while others will be fixed. If you're unsure, read reviews before buying one.
Material is also an important factor to consider when choosing a ski mask. Look for lightweight synthetic materials and avoid cotton. Cotton face masks tend to absorb moisture, making them cold and uncomfortable. If you tend to be hot, choose a thinner ski mask. Otherwise, choose a thicker one. A ski mask with a thicker face can keep you warm during the coldest parts of the mountain.