My only recomendation for size is get as large as you possibly get.
You can make the sump as simple or as complex as you want. At it's bare minimum it is just a low area for your system to hold extra tank water. It is also a good spot to put equipment you want to keep out of your main display tank.
At the bare min. you will have water enter the sump at one end, then have a pump at the opposite end to pump water back into the main tank.
If you want to complicate things you can add baffles to route the water flow where you want in the sump, add refugiums etc.
I like my sumps simple, in one end out the other. Stick a protein skimmer and heater in the middle. I also add a float switch in the sump so I can automatically top off my tank when the water starts to evaporate.
I'll post some links with sump info on them in a sec.