6x2x2 would be great.
If I would change a size on it, I would make it taller. They like to climb! With the taller size I would provide UTH and a radient heat panel on the warm side. 2 separate thermostats.

If you make it bigger from front to back, it can be a pain to clean. Of course, my arms aren't very long. I'm only 5'6". LOL