Though I usually agree with robthorn, I'll take the other side of the arguement on this one.
Size is not as important as some would say for a refugium. No matter what size the fuge is, it will provide benefits. To an extent, the larger the better. My 390 is being setup with a 44 gallon refugium. The key is much more what's in it. A DSB if very beneficial in a fuge for denitrification. Macroalgae is very beneficial for export of nutrients. Do a search through Dr. Randy Holmes-Farley's articles on RC and you will see the benefits of macroalgae in nutrient export. The macro must be continually harvested to ensure/allow continued growth. Macroalgae and rock rubble will provide a safe place for pods to multiply, therefore predators of pods must not be kept in the refugium. Also in an article on RC, an experiment was done where pods were passed through a pump from a fuge to a display tank. Over 96% of the pods survived the trip. In the real world, fish would rapidly consume the ones that didn't survive the trip, which is the purpose of raising pods in the fuge in the first place. As the pod population in the fuge increases, the number of pods that make it to the tank increases, which in addition to the macroalgae you raise in the fuge for feeding to display fish, now you are providing a steady source of fresh pods for fish food. It is important that sump/fuge flow be correct for pods to make it to the display. The fuge water must not pass through the skimmer. The skimmer will remove a vast majority of your pods from the water. If you have a recirculating skimmer, almost 100% of the pods that pass through the skimmer will become skimmate. Another benefit of the refugium is the additional water volume in the system, which allows for a higher bioload with less load on the system.
JMHO