I'm fairly certain these were 24" in diameter. The 30" diameter ones would be PLENTY big enough for even the largest of wheels!
The real beauty of this cage (when I had it) was that it was so easy to move, fit through doors, clean, and decorate. Since it was round, my gliders would literally run laps, jump from one side to the other, and make full use of the cage. If I ever build more cages, I'm definitely going with this idea, but I would probably build doors into the side as well. It would be tricky trying to deal with the curve, but I think it wouldn't be too hard to figure out