How many gallons do you want it to be?
You can do anything as long as you set your mind to it, but you want it to be useful enough that it can contain your return pump, and if you choose a skimmer and other types of filtration media.
So for arguments sake you want at least something that is close to 35-50% of total volume of water in your display tank. Reason being, should there be a power outage your sump will not overflow from your display tank draining down while your return pump is down.