Doesn't really matter the size of sump as long as it can hold your skimmer and other equipment. Ideally, I would fit the largest sump I can into the stand. Usually sump is around half the volume of display tank but if you can I would go bigger. Sump adds volume so bigger sump makes your system more stable than a small sump.
For instance, my 165g has a 90g wide sump.
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