Sizing a sump to your aquarium can be difficult. This calculator will tell you exactly what size you will need.
DIRECTIONS:
Enter your main aquariums dimensions in the first 3 boxes in inches.
Then enter the distance of the aquarium water in the main tank is above your overflow when the return pump is on.
If you are not sure what the height is(tank is not set up yet) then use 1 inch.
Finally, enter the percent of water missing from your sumps total capacity when running.(example: if your sump is missing 60% of its total capacity when running, then enter 60)
You will then get the amount of water that will drain from your tank if the pump shuts off, as well as the sump size needed.
WHY this calculator works:
When the power to your main pump shuts off, a certain amount of water will drain down to your sump, and potentially flood it if it is not sized properly. 
The calculator will tell you how much water this will be. It will then also tell you what size sump you need to have to accommodate the extra flow, while considering how much water is already in the sump while running. 
The minimum sump size recommended has 10% added to the actual minimum sump size. This adds a nice safety buffer to accommodate extra water from piping, etc.