I use the Critter trail 2 as the center for one of my tank setup...it was used for Ted E. Bear. It's empty now. See photo. It holds the Carolina Storm Wheel just fine and that wheel is 11" tall.
The really little cages would be just as carriers to the vet. The medium sized ones can be used for a hospital cage or a travel cage on trips. None of the small cages are really any good as a stand alone cage. We have a far way to go to get to the kind of cages they have outside the USA. And that's a real shame.