Ideally you want your sump to house 15-20% of the display tank volume while running. A 5'x2'x2' is 150 gallons so that would be 22.5-30 gallons of water in the sump at all times. What size is the 3' tank you have? Is it a 40g breeder?
Your best bet would be to use a 40 gallon breeder which is 36"x18"x16". That would allow you to run the 22.5-30 gallons with room to hold excess water from a power outage/pump failure. It is also short enough for you to be able to easily perform maintenance.
As for which return pump you should use depends on how much flow you want to have going through the tank. People like to have 7-10x hourly turnover of the tank including all of the methods of filtration whether it be HOB's, canisters, and the sump at the same time. Depending on the height of your stand it could be anywhere from 4 to 5 feet of head on the pump. So that is something you need to take into consideration. After you decide how many gph flow you want and the feet of head we can give you ideas for a return pump.