What size is the overflow? That's your largest factor on pump size. You ideally want a pump that'll handle a 6-10 times cycling of the tank's volume. Then you have to factor in the head loss.
Figure a 2400GPH pump will provide 1800 to 2000GPH flow on your tank with a 4-5 ft head. Your single drain should be at least 1.5" to handle this flow.