Cage Size?

Question
QUESTION:    I recently read that rats have happier lives in pairs or more.  So I have decided to get two rats. I currently have this cage: http://www.petsmart.com/product/index.jsp?productId=3057122. I know this will probably not be large enough for 2 rats, so what cages would you recommend?
  I currently have limited space, so i will not be able to accommodate a very large cage.
These are my three options of space for a cage:
13L x 26W x 25.5H
20L x 28.5W x 22.5H
16L x 26W x25H

(Also, i will be litter box training my rats)


ANSWER: The middle cage is the best size and will accomodate three rats just fine, which means that it would be ideal for two rats. In fact, that size sounds great for my three....I just dont have the space for my ferret nation and to be honest, the boys rarely use the bottom level and stick to the top so to free up space I am storing my FN and buying another cage for them, and the second cage is perfect.  Is this from petsmart?

---------- FOLLOW-UP ----------

QUESTION: Which of Martin's Cages would you recommend for 2 rats?
The Rat Cabin: 14L x 24W x 24H
The Rat Tower: 14L x 24W x 36H

P.S. In the future I will be purchasing the Midwest Critter Nation Cage(single)for them.

Answer
The rat tower is better and heres why:  I do not like when cages have half shelves and not full shelves.  I like when the cage has a full shelf plus a cage bottom to use. It is also so much more sanitary to litter train which I am glad your planning on doing that.  Rats are so easy to litter box train. Just make sure you do not leave any litter or bedding anywhere except in the litter box.  I just buy little plastic rectangle trays and use two, one inside the other. I fasten the bottom tray to the side of the cage so it doesnt tip over and I can remove the top tray to dump the litter. I would say the tray is about 10" long, 6" wide and about 3" deep is what I use for a smaller cage but in the ferret and critter nations you can use a small rectangle litter box used for cats.