What size prop should i use on a 115hp 16' boat?

There is no simple answer to this. It depends on the weight and hydrodynamics of the hull and the horsepower and rpm of the engine. It also depends on how you want the boat to perform. For a ski-boat, you want a lot of power "out of the hole", for a speed-boat you might want top end performance. As a general rule, boat manufacturers pick a prop that puts the same power into the water that the engine is capable of producing at wide open throttle. Check with the boat manufacturer or dealer for recommendations. Other than that, the only way is to experiment with different props.