I prefer the 10-12 inch hamster. The 8s are too small unless you use two, and the 15s and above are just sort of floppy. What's interesting is when you use a 8-10 inch hamster with a 8-10 inch rat. Then you can get some real power. Of course, you have to work hard at optimizing the box. Habitrail makes some useful equipment for this.

I am the Doctor, and THIS... is my SPOON!