I don't think that there is one right or perfect answer to this question. There are multiple variables that are going to need to be considered first. Is this just going to be a recovery sump or will it be used as the main filtration system? What will be in the sump and how much space will these apparatus take up (protein skimmer, filter pads / socks, pumps, return inlets etc.) What is the surface area of the display tank? How far will the sump be from the display tank? Both of these need to be considered to determine how much recovery the sump will be able to handle when the pumps are turned off. Just because you have "X" amount of gallons does not mean you all ways will need "X" amount of recovery in your sump. For example, If you have a standard 75, 90 or 110 gallon aquarium with the 48 x18 foot print the amount of water that back flows into your sump will be approximately the same. The consideration as to recovery in your sump is mostly going to be determined by how many inches of water will back flow out of the tank and into your sump when the pump is turned off. In this example, if each tank drained 2" of water into the sump, that volume of water would be the same even though they are different gallon aquariums. (48" x 18" x 2" is the same volume of water regardless of what the total gallon of the aquarium is)
What I do to determine what is the minimum size sump I am going to need is first check what the operating water level in the tank is and then what the approximate water level would be with the pump off. Lets use the 48" x 18" foot print as an example. If the water level dropped 2" that would be approx 7.5 gallons of water that back flowed into the sump. (48" x 18" x 2" = 1728 / 231 = 7.48 gallons) Take into consideration approximately how much water is in the lines as well. Next take into account what size sump you want to have, what it's operating level will be and weather or not it will accommodate the volume of water that will back flow into it when the pump is turned off. Now this is mostly a safety factor to determine so you won't have any floods and does not take into consideration filtration needs. That's a whole 'nother ball game.
Give a little more information about what your needs are and what the display tanks type is (reef / fish, etc..) what the tanks dimentions are, what type / style of pre filter, etc and I bet we can get a more accurate answer to the question. I agree with Joe's opinion that bigger is typically better but I also know that we normally have space limitations to deal with too. Post some more info and see what we can come up with...Joel