I dont really think a rule of thumb is appropriate here. The sump's main purpose is to hide the equipment needed to maintain water parameters. Therefore it can be as small as required to do that job.
Added benefit in having a larger sump is increased water volume in the overall system, which can allow for less frequent water changes or more livestock since your water parameters won't climb as fast (based completely on amount of livestock).
Of course a nice big refugium can be part of a sump as well, but thats almost a completely different component to your standard aquarium so doesnt completely apply to the OP's initial question.
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