Beth, I had similar situation as yours with limited space between back of tank and wall. I read up on protein skimmers, saw some interesting postings on this board and visited several websites as well, before deciding to go with the CPR Aquatic's Bak-Pak2 skimmer. The dimensions are 7.5" wide X 3" deep X 17.5" height. I have about 5 inches space behind tank and it hangs there very nicely, with couple inches to spare. There are 2 adjustments at the bottom of the skimmer that allow you to make sure it is positioned level/perpendicular to the tank. I don't have a lot of experience with other skimmers on the market, but I will say this, what I have read regarding air driven and venturi style skimmers vs in-tank models, I think you happier with the venturi type. I empty about 1/2 tp 3/4 inch of greenish brown nasty smelly stuff out of the cup each day .. and always feel good about that ! One chore I never mind is getting that gook out of the tank
I would imagine others on this board have used the CPR unit and would support us on this one !
I bought mine online and would be happy to email you the few sites that had decent pricing.
Good luck !
Broomer