I keep my females with litters in a ten gallon, per mama rat. They really do not do much except sleep and lay around and nurse the young while they have them.
I would only keep one weaned rat per ten gallon, though. But until the pups are weaned, a ten will suffice.
If you are just starting out, breeding one male and one female, you can leave them together for a time (I guess you;d have to if you wanted them to mate, lol.) I am not really experienced with actually mating rats, just raising the moms and babies. I cheated and got two already-preggo females from my local pet store.
-Jen. Back in the hobby after a hiatus!
Ball pythons:
0.1 normal; 1.1 albino. 1.0 pied; 0.1 het pied; 1.0 banana.