There are a few things to consider when determining the correct size for a return pump for an aquarium with a sump filter.
How far is the top of the tank from the bottom of the sump? Pumps have head pressure ratings which means the higher it pumps water, the slower it will pump. You need to find a pump that will give you the gallons/liters per hour you want at that height.
How large are the bulkheads and drain lines going from the tank to the filter? Water can only travel so fast through pipes so the pump doesn’t need to be faster than your drains can handle. Read the article on Waterflow for assistance. Also, you can use ball or gate valves on the pump output to adjust the flow of a pump to match your drains.
How many times per hour do you want to cycle the water through the filter? You will want an average of 4 – 10 times per hour depending on freshwater, saltwater and livestock load.
Another option is to run dual pumps in case one of them goes out. You will still have one operating pump until the broken one can be replaced. Just cut the water flow requirement in half for each pump.
Last, consider using DC powered pumps. They are easily adjustable with an included control panel and can be matched to your requirements above.