I have 2 overflow boxes, can't remember what the older one is but the newest one is an eshopps. They both are essentially the same design with a U tube and they work flawlessly. I would match the overflow box's capacity as close as possible to your output through your return/s. That way no bubbles will build up if the flow is too little,( some bubbles are OK). Remember equipment like chillers and UVs slow down a return pumps flow as do elbows in your plumbing. The best way to know what the flow is through your return is to run the return/s into a bucket and time it for 1 minute. Have the bucket the same height as the tank to account for head loss on the pump.