Bob24578, What you are explaining is information that isnt easily accessable for someone starting there first build. I have gathered info from many sources to put a plan together for my 75 gallon with a 55 gallon sump/refugium. I have toyed with having a second refugium. Size to be deternined. I am sure that I am not the only one who perked up when he or she read the word ECOSYSTEM. With that said, a thread detailing your thoughts on building said ecosystem would be worth its weight in gold. I am foaming out of the mouth just thinking about how such a thread would be priceless. My original thought was to build a very large refugium to home in addition to the macros to grow seagrasses. I decided to shelve that idea for awhile because the reef system is dauntung enough on its own. If you care to share your refugium experience it may go down as one of the best threads ever written for many reef hobbiests. Like I said, it would be priceless.I would say it really depends on your main purposes for having it. If you are primiarily using it for nutrient export -- then you should be able to grow enough macroalgae in a 20.
If you want to maximize critter diversity and populations -- then the bigger the better. In fact, in this scenario, you want different area types (ideally) like sand, rubble, rock, high flow, low flow, high and low light -- each will favor different critter types. The sky is the limit here! My refuge for my 120 mixed reef is a swimming pool in the basement -- connected to barrels -- and rubble zones in the main tank. It's an ecosystem.
You can also have a favorable zone specific for just the critters you want - like all rubble and macro for mysid shrimp -- or low flow deep rocks for copepods. A 20 can do this too...