12" tall is much better, such as in the RBI cages. Anything over 12 is a bit of overkill for ground dwellers like balls.

Here is my RBI 4x2, since you asked. I had to take everything out and put her in a tub for the winter while I tweak it, since the temps and such were all over the board, but I know how to fix it, so it's not a lost cause. It's a very nice cage for an adult and looks great.