I used the AquaFuge Refugium for my 37 Gallon for a while. It works great but I wish I had built a stand in the back to support the biggest one 25.5" x 4.5" x 12" . Even though this one also hang on, but I wouldn't trust the weight it will put on the backside of the tank. I sold mine recently as I moved on to a bigger tank with Sump in the stand.
The stand I am talking about for AquaFuge can be just like this one... (Forum will format it to little weired shape, but you get the idea
I would suggest the biggest one that the opening in your acrylic will allow. If you have a glass aquarium then I would say just go for the biggest one. I myself bought the mid sized one and once my caulerpa began to grow and it was filled with critters, I only wished I had bought the bigger one. In fact, I just moved everything into a larger tank and the fact that I got to build a bigger refugium was one of the most fun parts! So I say bigger the better (ladies, um, ignore that lie)...
i ran an aquafuge18 on my 46bow. it was aright for being a small fuge. mine ended up becomeing a large turf mat of hair algea, and provide a great home for copepods. i never fed my mandarinfish, the fuge provided good munchies for him to feast on.
the water level does tend to get a bit close to the top if you ever have a power outage, and the pump restarts. i have had it overflow on more than one occasion, due to that.