There are three primary dimensions used to describe inboard boat propellers, which are used to determine the performance of the prop for a specific application. Those three measurements are diameter, pitch, and cup.
The measurements are typically listed in the following format: XX" x YY" .ZZZ
XX is the total diameter of the prop measured in inches.
This is the size of the complete circle the prop makes during a full rotation.
YY is the pitch of the prop measured in inches. Pitch is the total theoretical distance in inches the prop would travel through the water when completing one full revolution if no slippage occurred.
ZZZ is the cup of the propeller blades, measured in thousands of an inch.
These three measurements do not tell everything there is to know about a propeller, but they do provide a good starting point for comparing two or more different propellers and how they will perform on your boat.