What size sump????

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If the sump is too small in relation to the water you are trying to push through it, you might run into trouble with bubbles getting pushed through into the tank.

That being said...the tank runs fine without a sump, so any sump is simply an increase in overall water volume, and a place to hide heaters, skimmers and whatever else you want to hide.

So I can't see there being any hard/fast rule about how large the sump needs to be...except to consider how much water you want to flow through it.