You can do a web-search to get an appropriate size. Most any ball of decent quality will be sturdy enough. Also give some consideration to the material. Some are smooth and kind of slippery. A "grippier" rubber works better. Then, there's color. Steer clear of the pinks and turquoise balls.
FWIW, a yoga mat is a good accessory to use with the ball. Easier on knees, etc., and feet and ball stay planted instead of slipping when doing crunches, planks, etc. For those, Tar-jay or equivalent is fine... figure $10-$15.