I'd get a 40g long to make a sump for a 20g tank.
Seriously.
It's not so much about the size of the sump, it's that your intended gear will fit in it. IE, chillers, skimmers, return pump, fuge, UV sterilizer, Nitrate reductor coil, ETC.... So you should put some thought into what is going IN the sump to make sure your sump is large enough.
You wouldn't want to get that really sweet skimmer only to find out it won't fit in the 5g sump. A 10-20g would suffice. A 30 or 40 would offer even more choices.