Most No Parking signs must at least be 12 inch in width and 18 inch in height and they are mounted at a height of 6 feet. This is the minimum size requirement as per MUTCD Section 2B.03. In practice, we suggest you use bigger 18” x 24” parking signs because a bigger sign can be seen from a greater distance and over large trucks and SUVs. Every state can have their own requirement on sign size and sign mounting height.