http://www.rattycorner.com/odds/calc.shtml
Rats absolutely need a large cage. Much larger than a gerbil or hamster (which, if you get one of these, do not put it in a glass tank, it is extremely unhealthy). Also, rats cannot be put in a cage with wire flooring. It will cause foot problems (specifically, Bumblefoot) and even leg breaks to have your rat walking on wire.
Also, please avoid buying from a pet store. The animals are unhealthy and they are rarely, if ever, handled. They are not what a new owner needs (or even an experienced one). Adopt or buy from a breeder.
I'm sure they could even compare the gerbil to rat advantages/disadvantages, if you wanted.
ewww, not a rat, ewww
Oh, you're helpful.
If you get a baby, it can be kept in a smaller cage for a while.
I can't give you exact measurements, but I can tell you that it needs plenty of room to burrow about, and have room to exercise within the cage for when you're not around.
certainly not a cage that is smaller than they are.
probably one they can turn around comfortably in have a bit of moving space.