Rats need floor space that is more square than rectangular. You should also provide for a second level for them to do their rat thing. Something like 24” long x 16” deep x 24” high.
As I stated in my first post, the cage described would work, particularly given its three levels, but the 18” seems a bit cramped for the way rats like to run around at times. However, another consideration is the size of your rats. Smaller ones I’m sure would do fine in the cage described in your original, but I’ve seen some really large rats that I’m sure would be cramped. I am one to think more space is better, particularly for animals that are quite active. Just some thoughts.But the one above is bigger than that and also 3 level instead of two level. Can you help me understand what you mean when you say that the one above is too small?