Props are specified by length and pitch. A 10x7 prop is a 10" prop (measured tip to tip) with a 7" pitch (one revolution will "screw" the prop 7" through the air). I believe Metric prop specs are in mm instead of inches. A replacement prop should be of the same length and pitch for similar performance and current draw. If you over prop an electric setup, you'll draw too much current and fry your motor, speed controller, and/or battery.