I would just breed rats, they are much easier to raise and they don't smell so bad.

It takes awhile for rats to bulk up to adult size. At four months old I'd say they are around medium size. Small I'm not entirely sure, maybe 2-3 months? They are roughly the size of mice at 1 month. Jumbo.....you're talking 6 months+

I keep my rats and snakes in the same room. I've had absolutely no problems other than all of my snakes LOVE feeding day How do they know it's feeding day? I take out everything in their tub, give them a few minutes and put in their food, I can hardly get my hand out before all six strike and coil their food!