If you look at a 12-inch boat propeller, you might be surprised to see that the propeller blades aren't even 6 inches long. That's because a 12-inch propeller isn't a propeller with 12-inch blades. The size of a boat's propeller is the diameter of the circle the blades make when the propeller turns. Since most "stock" boat propellers have three blades, measuring the diameter of that circle isn't simply a matter of measuring from one blade tip to another--it takes an extra step.
Measure the outside diameter of the outside of the hub of the propeller, and divide by 2. This is the radius of the hub.
Measure from the outside of the hub of the propeller to the end of one of the propeller blades to determine its length.
Add the length of the propeller blade to the radius of the hub. Multiply the result by 2. The result is the diameter of the propeller.
Remember that the edges of a plastic propeller might be sharp, so be careful.