Use as big as you want. The sump is really just for equipment and water volume. I use a 20 gal
water) for a sump and a 25 gal
rubbermaid container(20 gal
water) for a fuge. Both work fine. I actually like the glass tanks better but I needed a drilled contaner for the fuge and did not want to drill glass so I opted for the rubbermaid.
Tanks have a smaller footprint so space is not as much of an issue. I think most say to usa as big as you can. I went with a 20 gal
sump because it would fit under my stand at the time
Keep in mind a 19 gal
won't actually have 19 gal
of water. You need to leave room for when the pumps are off and the lines drain.