A rat cage should be as large as possible to give the occupants room to run and play. The minimum size is 12 inches wide by 24 inches long by 14 inches tall. Aquariums with mesh tops can be used but should be even larger for better ventilation. Wire cages are the best for ventilation and rats can climb them. Make sure though the wire spacing is not big enough for the rats head to go through and the floor is not wire, due to feet and limbs getting caught.
What to put in the cage..
Ladders, Branches, other climbing toys
Milk Containers or Oatmeal containers
Old shirt or sock
Editors Note: I have my rats in a 2 20L Aquariums. One for males, one for females and they do quite well in them. Also when putting in toys, BECAREFUL, this gives them advantage to get out of thhe cage, my female breeder rat has escaped 2x on me, now I put cage locks on her cage!
Some info gotten from Critters USA by Fancy Publishing