Larger overflow boxes will handle more flow/hour. It will depend on your return pump or any possible future upgrades. For instance, on my 75, I used to run a U-tube style overflow box that was rated at 1200 gph. My return pump was putting out about 900 gph. Because of this, I would regularly get air bubbles in the U-tubes and have to re-prime them. With U-tube overflow boxes, I'd recommend not using one rated for much more than your return pump, for this reason.
With CPR overflow boxes, it doesn't matter. I'm currently running a CPR overflow box rated at 2200 gph. I'm still using the return pump that's putting out about 900 gph. I've had no problems at all. However, this CPR overflow box would allow me to upgrade to a much larger return pump, a much larger tank, or whatever.