I think the R-690 is probably fine for young rats but as they get bigger, the 14" depth is a bit of a squeeze. Rats can grow up to 11" long (not including their tails) so movement can definitely get a little restricted. I'm also not a fan of narrow cages. Rats are terrestrial animals, not arboreal. They live on the ground, not in trees. I think it's important to give them floor space to run around. I wouldn't personally use a cage narrower than 30" regardless of how tall it was (wider is even better). If I were in your position, I'd probably find a different place to put the cage that allows for a bit more width.
-edit- ...and if you purchase a Martin's cage, be sure to get the powder coating. The front doors are big enough that I don't personally think that a flip-top lid is necessary, especially when purchasing a taller cage. The flip top can just make things more difficult if you're hanging accessories like hammocks and Space Pods from the top of the cage.
