I dont know if you can rate a fuge for a tank size, allways biggest is better. So IMO, use the biggest one you can fit on the back.
They make two sizes with the PS, a small and a larger one....19-1/4" x 3-3/4" x 14-5/8" and
23-1/2" x 3-3/4" x 14-5/8"
I rigged an aquafuge ps small to eclipse sytem,,, dont ask me how,,but i did it! Check out www dot cpraquatics dot com they have all info you will need and they also have a very useful forum for us aquafuge owners! GOOD LUCK!
I hooked up the medium sized CPR to my 90 gallon and it seems to do fine.
I have it loaded with algea that looks like a bunch of little grapes. Before i had it hooked up my nitrates were getting on the high side (no matter the number of water changes) and after installing it my nitrates have dropped a lot. I have many neat little mysis shrimps and other bugs living and swimming in there. I have a mandarin fish and im sure he appricates the extra food.