I agree with ragin cajun, I never put any dividers in my sumps, and size is whatever you want. Whatever size fits under the tank, if you want your sump under the tank.
But even that is up to you, I put some sumps below the tank, some off to the side or some above on a window ledge, you can even put a sump in the basement if your tank is on the 1st floor, in order to reduce pump and water flow noise, or to help maintain a cooler temp.
I have also used a 1500 gallon pond as a sump for a 55 gal tank.
Your only restriction may be the holes already drilled in the tank, that determine pump size.
I always use all drill holes for the overflows, but never for returns. I like to have plumbing above the tank for flow of return water , to provide agitation at the waters surface.
2 overflow holes allow for more pump age per gallon. But that is strictly personal preference.