If you want a Wavemaster
http://www.centuryfitness.com/webapp...18&crumb=13501, I would suggest the XXL size.
It will still move around some, but won't be as easy to knock over.
We use these at the studio where I train, and the smaller ones get kicked over all the time. XXL's will go over as well, but not nearly as often.
I am not all that big (~170 lb.), and both sizes tend to walk, especially with kicks.