the size of pump you'll need is restricted by your overflow. Both the overflow surface length and size of plumbing inside the overflow (diameter of the drain basically) will restrict the amount of water that can exit the tank into the sump. You don't want to have water pumping into the tank faster than it is exiting.
For example, I currently have a mag 12 (1200gph) running on my 90gallon tank. I have 12" of overflow length and a 1.5" pipe running into the sump. I find that even with 6-7' of return plumbing to the tank its almost too much for the overflow to handle. Previously I used a mag 9 (950gph) and it was fine however I went to the mag 12 because I was running a chiller at the time and the extra length of return plumbing slowed down the pump.
I'd say you can likely run between 600 and 900 gph through your sump. You'll have to add powerheads to the tank in order to increase your flow as just having the return pump will not be near enough flow.
Perhaps if you can provide more information on the diameter of the plumbing in the overflow as well as length of the overflow (or take a photo if thats easier) we can give you some better suggestions
