Peanut. As recommended to me my Izzie's masseuse and her chiro vet. The peanut ball allows the dog to stand naturally instead of hunched up like on a regular ball. Izzie has the teeniest one from fitPAWS but fit to the largest size of dog you will be putting on it, smaller dogs can use bigger balls but bigger dogs can't use smaller balls.
Okay that last bit sounded better in my head...