Probably as many opinions as people on this one. You can use a calculator like this http://www.reefcentral.com/calc/hlc2...=1&Entrances=1
to figure out what your flow will be with one of the more popular pumps on the market.
the size of the sump/fuge determines the size of the pump more than the size of the tank. You are not trying to achieve the desired flow in the display tank with the return pump. You are only trying to circulate water for treatment.