A sump is just used to provide increased water volume for your system (things can go downhill a lot quicker with less water) as well as a place to "hide" things such as heaters, skimmers etc that you wouldn't want to have detracting from the display tank.
While I'm not aware of any calculations for "correct" sump size, it doesn't mean there isn't one out there. I just got the biggest one that would fit in my stand
