For the overflow return, knowing the inlet flow, the height between the overflow nozzle and the top hat relief ( max driving force for return without overflow out top hat) , pressure drop from pump discharge to fill inlet, & pump head curve, one could predict the line size required to prevent overflow out the top hat.
To minimize the increase, just run the overflow return from the chosen height to ground level and use a trench back to the pond. My guess would be a 4 or 6 inch might do it.