[Iraf]

I haven't personally built one but I can say without much doubt a 10g would be to small, the big thing you have to figure out is how much water is going to drain off your display tank into the sump in case of power outage which is dependent on the placement of your overflow, most skim the top 1.5-2" of water so thats probably a good 8-10 gallons there of overflow, plus you will constantly want to maintain your sump at about 50% full or close to to help with heat dissipation if you run a submerged pump as alot of people do.

There are guys on here with alot more experience and alot better advice than this but just my $.02 worth

Personally I wouldnt go smaller than a 20g sump and maybe a 5-10g for a refugium