I actually broke down my 30L reef tank a few months ago and gave it to my brother to use the tank as his sump/refugium and the live rock in his reef. A 30L is great because it's not as tall as a 29 so reaching into the sump to service or install pumps, reactors & skimmers is easier. It's also not to long so he still has room to keep stuff on the sides of the stand.
You may find you have more room in the stand for something larger but if you go any smaller you could be disappointed with how easily a sump can run out of room. The BEST time to buy a sump is when Petsmart has a dollar per gallon tank sale.
Oh, a refugium is also good for mild pH stabilization. When the lights are on at night the plants use up co2
from the display tank and produce oxygen. That keeps the co2
from dropping at night. It also is a "refuge" for small animals like copepods and mysid shrimp to breed freely without getting eaten by your fish. The larvae get pumped into the main tank as food for the fish. That "refuge" is the basis for the term refugium.