if you can fit 55 gallons, i personally would do 55 gallons. you dont have to have a sump this large but the bigger it is the more water volume you will have and more stable the tank will be. a larger sump also means it has more room to fit equipment ( a nice large skimmer, carbon/phosban reactors, heaters, even a refugium area and so forth )
if another tank of this size wont fit underneath then a used 20 long or 30 gallon tank would make a very functional sump. just make sure it holds water before you silicon in baffles and make sure your skimmer footprint will fit in the section made for it.