The balls that I see used in Treibball in Germany are air filled 'fitness balls' or 'exercise balls' (Gymnastikbaelle), 22" to 34" (55 cm to 85 cm) in Ý, depending on the dog's size. Roughly the height of sheep! I searched for them on ebay in the US and found plenty.
But unless you are competing, you can of course adapt the rules, invent new ones, and use a smaller ball if that works better for your dog. As you say, any ball bigger than the dog's mouth should work.
In Germany some people use 'piglet balls' (Ferkelbaelle), smaller than the exercise balls. They were originally developed as enrichment for bored piglets in stables, and are very sturdy.
Btw, the fitness balls used for Treibball are very popular for people use in Germany and Switzerland. You can sometimes even get cheap ones at the supermarket around the corner. The quality ones also make great seats for long hours at the desk. They make for an upright posture without rigidity. Sitting on them one moves subtly all the time to keep balance. Prevents RSI and muscle tensions. No slumping while seated on a fitness ball. Might want to remove the dog saliva first, though!
Chip vom Dog Pound, border terrior, Director of Rodent Control, CGC
Elly von Rescue, GSD, CGC, HI(C)
Captain Jack, Mama Blue, Little Orca, BŁbchen, Tux, Cherub (pocket lions)