2013 Pikes Peak Open Aerodynamic/Wing Clarification
The furthest point of any rear wing or device cannot extend more than 12 inches from the furthest point of the body on the rear of the vehicle. The highest point of any aerodynamic device or wing shall be limited to no more than 18 inches above the tallest point of the cab on the vehicle.
Source: 2013 PPIHC Rulebook - Pikes Peak International Hill Climb
Effect of the height of the front and rear wings on downforce and drag
The interaction between the rear wing and diffuser leads to a decrease in total pressure behind an F1 race car. (Source: Driven to Simulation - Ansys)