The overflow box gets mounted on the inside back of your tank, such that the drilled hole is inside of it.
They are mounted near the water surface and extend about 6-inches down from the top. (The positioning of the drilled hole should be determined with your overflow box in hand so you get it in the best spot.)
The "teeth" are the cut out areas that prevent debris from entering your drain. I would definitely recommend an overflow with a screen to minimize critters from getting into your drain. (A plugged drain would result in a flood in short order because your return pump would continue to pump from your sump and water would not be flowing into the drain like it should causing the display to overflow.) Also the design of the teeth in the weir can impact capacity...so going larger provides some safety factor for the notches.
Typically you could get by with a return hole that is smaller since that is flowing under pressure from the pump, rather than gravity. However in practice I think a lot of returns and drains end up being the same size because it is easier to just use the same bit for both.
This one's specs look pretty close:
AO-BLK700-OV Premium Aquatics - AO-BLK700-OV Aquarium Supplies
But I would get this one instead because it has a little additional capacity and would operate more quietly:
AO-BLK1200-OV Premium Aquatics - AO-BLK1200-OV Aquarium Supplies
This one is for a tank that is a lot larger:
AO-BLK2400-OV Premium Aquatics - AO-BLK2400-OV Aquarium Supplies
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