The bigger the better when running a true refugium most hobbiest do not run a refugium they run a catch all.
What is a refugium? A safe place where life can flourish without preadation...the end! You can have many different kinds of refugiums so knowing what you want your refugium to do, will determined the size needed.
If you want it to help with supplying your DT with snacks thru out the day/night and as a nutrient up take then minimum 25% of system volume. I would go larger and use a stemmed/rooting macro algae with sand and some open rock structure's with as little rock touching sand as possible. And use high flow thru your refugium or the snacks will never reach your DT on their own. And also the algae doesn't care if it high flow it will still remove nutrient's those who say different just like slow flow thru their fuge nothing else. Lots of light with the algae I ran a 250 watt pendant w/6500k over each fruit ben with harvesting about 1/3 to 1/2 of a five gallon bucket out of each per month/month and a half. I know you will be using LEDs so some trial and error will be needed in this area.
Last we spend $ 1000s on lights, DTs, pumps, skimmers and pennies on a refugium so when spending pennies compared to how much we spend on other equipment do you really think it works...it doesn't FYI
There is only two way's to tell if your refugium is working and in balance with the rest of your system when used as outlined above. One is the quality and amount of algae harvested each month and the other is...................................