I got a pretty generic one from Kmart for about $15, that has no trouble holding my weight. They say the way to check the size is to sit on it and make sure that your knees are at a 90 degree angle. If you are sitting lower then that it needs to be bigger, if you are sitting higher then that it needs to be smaller. The one I got had a size chart on the back of the box to help you determine what size to get, plus in the instructions, it says you can determine how big it is to a certain extent, by how much you fill it, as long as you don't fill it beyond a certain width. I really like mine, I use it for lots of great leg toning and several different arm toning exercises.