If you use a submersible pump, you can have a sump use up nearly all the available space. Most LFS
can arrange to build a cheap glass tank to any specified size.
Bigger is better. For safety, you'll only want run the sump 1/2 full...or a least empty enough to hold water that flows backward from the tank in the event of power failure. There's another discussion on this somewhere...
I've got a 2' x 16" x 18" sump wedged into a 2' x 2' x 2' space. Plan, plan, plan and plan more...then do. To the inch, things are spec'd out so that I can remove and access everything.