How big should my rat cage be?

  I bought one rat yesterday, shes in cage that is 20x15 inches.

I'm aware that its best to keep rats in pairs or groups so I'm planning to get her a bigger cage next weekend and introduce a new cage mate.



So how big should a cage be for one rat?

And what size should it be if i want two?



Thanks! (=
Always go for height versus floor space, they love to climb. About 1.5-2 cubed feet per rat is the popular standard.
I have heard absolute minimum of 2 "by 2" by 2 " for a pair of rats, but more is probably better. Just make sure they have room to enjoy themselves and for all their toys.