Generally the extreme of any case proves the point.
Canadian your right that you don't want a overflow that's too small. If I have a flow rate of 3000 gph and a 3" linear overflow I'll be sucking a lot of water from under the surface and potentially not getting a lot of surface skimming.
sphelps you're also right that you don't want and overflow that's too big. If I have a flow rate of 1 gph and a 10" linear overflow the surface of the tank will get build up like crazy.
To small or too big can be bad but like sphelps said I'd rather error on the side of being too small and sucking out water from below the surface then have one that's too big and have some nasty surface buildup.
It all depends on how much flow you have coming from the sump. Some people prefer power heads for the majority of flow so the sump return pump is pretty small in which case you wouldn't want a really big overflow.
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