All animals, no matter what type, have a minimum space requirement.   Ideally they should have more space than they need, in order to stay happy and interested in their surroundings.  For two rats, an ideal home is a large bird cage or one of the specially designed hutches for small animals.  If you like to be able to see your little pets a bit easier you can keep them in a large fish tank with a solid wire top on it, however this is a little harder to clean.

 

 

See how to fit out your new cage.