A lot of factors come into play with changing prop sizes to determine the best power output. For example, you have some engines where more output is achieved at lower RPMs. Then, there are the characteristics of the props themselves.

Generally speaking about props, it is true that a larger diameter prop is more efficient than smaller diameters. Looking at the bore and stroke of the engine could give some idea of whether it may like larger or smaller props. In the end, it always comes down to a fly off between your props, engine, and the airframe. Some people forget about the cowl or cheek area directly behind the prop, and that figures into prop performance.

Almost nothing can be learned while taching the engines on the ground as this has nothing to do with the environment of flying at some airspeed. You can measure hover power this way though.