The return pump should always be rated higher than the overflow. Are you drilling the tank or using an overflow box?
Also, it needs to accomodate for the fact that the pump has to pump the water up a few feet. You'll need to measure this.
There's a handy app/tool online to calculating what size return you'll need. I'm at work right now so I can't look it up for you but if nobody knows it I'll try to post it later.