Such sign shall be at least three feet by four feet in size, consist of sturdy and serviceable material containing a white background with black letters and be placed in a location visible from the most commonly traveled street or highway upon which the property fronts, or in the case of a corner lot on both streets, but in no case more than 20 feet back from the front lot line. Such sign shall read as follows, in legible lettering with the heading at least five inches in height and the content at least two inches in height: