I think they're actually using a 55cm ball. I don't have GS Legs yet (ordered it last night, wahoo!) but the other ones are all 55cm. Since all the crew are Cathe's size, and Cathe is 5'2", maybe that will help you gage what you need?
Also try performing a search of the forum for the correct size for your height. I know we've posted links to various charts on choosing the right size for your body.
HTH!!