More important is that you get the right height stability ball- if you look on some of the site, you should find a chart that will give you a good idea. If you are a little learly of going straight to a regular ball, I bought what I affectionetly call a 'peanut ball' at WalMart for some exercises. Its, well, peanut shaped- like two smaller balls got stuck together so there is a little bit of a valley in the middle. With the 'peanut ball' (there has to a technical name....), you only have instability in one dirrection (front to back- not front to back, side to side, side to front, side to back, etc that you have with a regular stability ball).
I also bought a strap kit for my regular, round, stability ball at WalMart- it give you options for resistance training with the ball- plus its easier to position the ball with using the straps...I admitt I'm not the most gracefull person. I use my round stability ball at my computer sometimes- its a good time to work on your core without really thinking about it (you could do it while watching TV....I happen to watch TV on my computer

). One word of caution about using it as a 'chair'- I got so comfortable using it for awhile that I forgot I was on the ball when I stood up to reach something...unfortunetly balls tend to roll away, so I got a surprise when I sat back down

Also, you can get a stability disk you can put in any chair- great to take to work or such...if I could steal mine back from my Mom, I'd let you know how well it works for me.
Another tip on stability balls- make sure you have an electric pump of some type....takes forever with the little manual pump they typically come with. Also, make sure you measure it so you know you have it fully inflated....put a little more air in then you think since you'll lose a little putting the stopper thingie back in.
Have a BALL!
