Whats a good size sump and refuigium for a 55 gallon.
Personally for that sized system, I wouldn't go any smaller than say 20-30Gs. There are lots of people that even have sumps and 'fuges with more water volume than the display tanks themselves! :o
Go with what you feel comfortable having, and as big as you can with the space you have availb. to you. ;)

W.