The area of a sign is the smallest circle, square, or rectangle that encloses a sign face or the largest plane of a three-dimensional sign. The area of a double-face sign is the area of a single face. The area of a multiple-face sign (other than a double-face sign) is the sum of the areas of all sign faces. Only the sign portion of a structure, material, space, or device is calculated for the purpose of sign area, except that the area of a cabinet sign or sign in a frame or border shall be based on the outside dimensions of the cabinet, frame, or border. The area of multiple signs on a structure, material, space, or device is the sum of the areas of all signs. The area of a sign with multiple messages is the smallest circle, square, or rectangle that encloses all of the messages.