The 3/4" is fine going from your overflow to your sump. I'd use 1/2" from your sump back into your tank, and I really wouldn't think you'd need much more than 150-200 gph
. At 200 gph
you're doing 20x your tank volume, but it really does depend on what you have in the tank.
There's a pretty good pvc
flow rate chart at http://flexpvc.com/WaterFlowBasedOnPipeSize.shtml
. It's for flexible tubing, but it should be somewhat similar for standard pvc
As always, whatever you put on the return should include a ball valve. That way you can adjust your return to make sure that your overflow can keep up.