Sign area measurement. The area of any sign shall be computed as the product of the largest horizontal width and the largest vertical height of the lettering, illustration, display or background. This shall not be construed to include the supporting members of any sign which are used solely for such purpose. If the letters, illustration, display or background is attached directly to the face of a building, the height or width of the sign shall be the height or width of the largest letter, illustration, display or background, whichever is the greater. For signs with two display faces, the maximum area requirement shall be permitted on each side. Signs with more than two display faces are prohibited.