In general you should use the largest diameter prop that your boat is
capable of swinging, and that is determined by tip clearance. If tip
clearance from blade to hull is too small, the shaft loading becomes
asymetric and will result in excessive wear and vibration. The normal
clearance recommendation is for no less than 20 to 30% of prop
diameter. On the other hand, using a prop with too small a diameter
will result in greater slip under heavy load. If the used prop is a
bargain you could always keep it as a spare if it doesn't perform to
your liking.