Enough…
When I went to school, 13 inches was quite a lot. (Haaarrr, haaaarrr!) Now, filth aside, I wouldn’t bother with the hamster ball at all. Just go with the omni wheels. I like the orientation of the wheels in the cartoon: the axles wold be perpendicular to the side of teh 'bot. That’s quite unusual. It would work perfectly, though.