Very simply (and neglecting internal drivetrane considerations):
Power In = Power Out
Power Out = Thrust Power - power consumed by drag
The ratio of of Thrust power divided by applied power, is efficiency.
Your goal is to maximize thrust and minimize drag, thus maximizing efficiency.
To achieve increased thrust, you can either spin a given prop faster, or grow the size of the prop.
Now, with increases in a prop's angular velocity (rpms) and therefore the relative tangential velocity of a given radial element, you inherit torque drag in a directly proportional relationship to the 2nd power. Conversely, increasing blade area to achieve the same goal only increase torque drag linearly. Therefore, it would lend itself to reason that a larger prop turning slower would yield greater efficiency than a smaller prop turning faster - all with limits, of course.
As an additional note, here's a handy rule of thumb:
Doubling your blade area, doubles your drag.
Doubling your blade velocity, quadruples your drag.