I think it really depends on what you're going to run in the sump. For example, if you're going to have a refugium, skimmer, reactors, internal pump, etc. you're probably going to need a large sump. If you're not going to have a ton of gear and a fuge, you can get by with a smaller sump. Make sure it's not so small that it can't hold all the water that overflows when you shut your return pump off. With that said though, extra sump space is nice to have.