Smallest tanks that you have with a refugium?

I have a friend who is talking about how he runs a refugium on a 10-gallon tank and it sounded odd since many of my own refugium tend to be almost double that size. That got me thinking about it all, and I was curious if many it was just me not running across many nano-tanks who are in the reef realm. What are some of the smaller tanks that you all run that have a refugium?