With a 120 gallon tank, one inch in height equal ~5 gallons of water. If your overflow box is 2 inches below the top edge, then potentially when the pump shuts off, the water will continue to run into the tank until 10 gallons has drained out. Your sump will be half full. That means, a bare minimum in this example would be a twenty gallon sump. The overflow may not allow that much to drain and you may get along with less. The point is, you need a sump that will hold at least twice the volume of water released to it in the event of a power failure. 2nd, the width and length of the sump must be one that can accommodate the equipment you plan to put in it.
Mike