A couple of things I like to consider-
1- will the unfilled volume of the sump during operation be sufficient to contain the draining of any tank & plumbing water to static levels after a power outage or shut down?
(otherwise, it will go on the floor!)
2- is the volume of the sump sufficient to to allow for the things I want it to contain?
ex: skimmer, return pump, filter media, macro algae, fleece roller, socks, rubble/ceramic media, specimen acclimation/isolation chamber, etc.
3- is the volume sufficient to keep velocity of flow slow enough to allow sufficient media contact time & prevent other problems from too high of a flow?