What you need to measure is sq. ft. (or sq. in.) of floor space. http://www.ahc.umn.edu/rar/cagespace.html
I think Dan suggested using alfalfa pellets for bedding. It works really well.
As far as killing them goes, I wack them. Hold them by their hips with your thumb on their belly, your finger behind their hips and their tail down the palm of your hand. Wack the back of their head on the edge of a counter. Quick and painless.