Pretty much anything that the rats can't get out of can be used to breed them in as long as it's big enough. I keep a 1.3 in the concrete tubs that measure about 26" x 20" x 6".

My rats breed best at 65-80 degrees Fahrenheit and tend to slow down or stop out of that range, so if you want them to keep reproducing during the winter you'll need to heat them.