Caitlyn usually chooses a women's size 6, her skivvy preference for her wardrobe is a size AA. She is wearing a size A in Black, and B in Natural.
Mara usually chooses a women's size 6-8 and is wearing a size B in Hot Choc.
Helene usually chooses a women's size 10-12 and is wearing a size C in Green, and Navy.
More size information here.