A sump is just something to gear like pumps, filter media, heaters and other stuff in, you are only limited by your own imagination, you can also plant them with aquatic or emergent plants as a refugium style.
But...the most important factor for me in any sump, is easy access for maintenance.
I hate putting sumps in a cabinet under a tank, because often times you need to be an expert yogi to get access. I also want plumbing to be easily accessible if I want to change something.
I know aquarists that put sumps in adjacent utility rooms for ease of maintenance, and so they are out of view. I know people who put sumps in a basement, even though the tanks are on a floor above.
I have used a 1500 gallon in-ground pond, as a sump for 55 and 75 gallon outdoor tanks, to help maintain temp stability outside.
The more room you allow, the more creative you can get with filtration methods.
Below, a 55 gallon tank with a 1000 gallon pond for a sump.
At the moment I'm using 2 commercial type sumps boxes on my 180 gal, because they came with the tank, but my plan is to modify them at some point.
One is an algae scrubber on one side, and the other side holds the pump and floating aquatic plants.
The other sump has a biological contractor (also a bit over grown with algae), and contained submerged bio-balls.