I purchased an 8 lb medicine ball a few yeasr ago to use with Boot Camp. Cathe uses an 8 lb medicine ball in Boot Camp as well. I use it for Core Max and several of her other w/o's as well. Some people may use a 10 lb. med ball but I prefer the 8. You still feel the burn!
FYI: If you get the 8# and find that it seems a bit too heavy, just shorten your lever (ie: bring your arms closer to your body) and it will seem lighter.