| Bigger is better. The more water you have, the more stable it is. For a refugium, you want at least 10% of the display tank's volume, so at least 4 gallons. Again, more is better. A common way to have a sump/refugium in the same tank is to put the sump with equipment and everything on one side, and the refugium with a deep sand bed and macro algae on the other side, with the return pump in the middle. You take the drain from the overflow and split it, putting a ball valve on both outputs. Run one pipe to the fuge, and one to the sump. A 20L tank should work fine giving you the space you need. | 
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