Only 2 dimensions of a propeller are of interest to a boat owner: diameter and pitch. These metrics are usually stamped on the propeller's hub with diameter being the first field and pitch being the seconds. Example: a reading of 10-15 would mean the prop has a diameter of 10 inches and a pitch of 15 inches.
Diameter is the measure of the blades "tip to tip". If a propeller is 3-bladed, you can measure the distance from the tip of one of the blades to the center and double it to get the diameter.
The pitch of a propeller is the angle at which the blades are attached to the central hub. In the previous 10-15 example, ideally the propeller should travel 15 inches through the water each time it makes a revolution. This is an ideal situation, factors such as slippage affect it.
The blade area of the propeller determines its load carrying capacity. A 2 bladed propeller is used for high speed running with a light load. A four-bladed Johnson propeller would be installed on a boat that is expected to work under very heavy loads (i.e. a heaving fishing boat or a houseboat). A 3-bladed Johnson propeller is a good compromise between speed and load.
