The dead pedal was designed off of the dead pedal in a Ferrari. Its large because of how much space there is between the left side and the brake pedal. It give you an extremely rigid place to put your foot during extreme driving. The brake and gas pedals require 2 1/4" holes to be drilled in the pedal. Unlike may other kits that only clamp on to the pedal. Ours are very secure and dont move around. The dead pedal requires 4 self tapping screws that go into the sheet metal under the carpet liner making it very secure.
As for after market mats. I dont know if the pedal will fit or not. With the stock mats you can either cut a small section from the mat about a 3X4" section or attach the pedal over the mat. The only problem with that option is the mat cant be removed then.