sump size for 220g tank

[xtreme]

I would go as big as you can fit/afford. The bigger the sump the better. More water volume means a more stable system, more space for skimmers, reactors, heaters etc. I have seen some tanks where the sump is actually bigger than the display.

More water does have a down side though, it will cost more for salt and electricity to heat/chill the water. But to me the benefits of a larger volume sump outweigh the disadvantages.