With any sump (besides the obvious reason that bigger is better) its all about whether or not the sump has enough room for the equipment (pump, heater, other paraphernalia) and the ease of doing maintenance, like, how easy it is to get your arms in.
I would imagine the footprint of a 20 long, and its height would be better for both eqipment, and allowing elbow room for ease of maintenance.
I have drilled tanks as small as 20 gallon to add to my lines of tanks (I will often combine 4 or 5 tanks on 1 sump. Its easy to drill, and run PVC lines and add more tanks, without ever having to cycle them.
Below is a drilled 20 gal, the sump is a 75 gal, for 4 other tanks, beside the 20.
One caveat, it is harder to drill thinner glass panel tanks, than larger thick glass ones, Drilling slowly and carefully, with plenty of cooling water helps.