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Rules of thumb for propeller design...
1. Long, narrow blades are more efficient than short, stubby ones, for the same reason that sailplane wings are more efficient than an Ag Cat's: higher aspect ratio means lower induced drag.
2. The longer the prop blades, the lower the rpm at which the tips go supersonic. The longer the prop blades, the more difficult the structural design problem. The longer the prop blades, the more likely are ground strikes.
3. To dissipate more power, the compromises inherent in balancing items 1 and 2 require that you add more blades.
Successive Spitfire variants received ever more...Show more →
The other alternative to more blades is wider blades. You see this a lot in piston aircraft where they have been repowered. The new motor has more power, or torque, but the propeller diameter can't change. The choice is more blades, or wider blades. Wider blades is easier, faster, and cheaper to do than re-engineering a new propeller hub system, especially for moveable blades like adjustable pitch propellers, or rotors.
To the original poster. The term for the length of an airfoil is chord. I believe it is pronounced as cord. The length of an airfoil is the width of the wing, so, you would use wing chord to describe the width of a wing/propeller/rotor, at any point along it's length.