One real estate sign which shall not be larger than two feet high by three feet long on one lot advertising the sale or letting of only the premises on which it is maintained and set back not less than five feet from any street line and not less than 10 feet from any property line which is not a street line, except that, where acreage available for or in process of subdivision is advertised for sale or let, one real estate sign not larger than two feet high by three feet long will be permitted on each 1,500 feet or major fraction thereof of street frontage, subject to the setback provisions hereinabove set forth, or, in lieu thereof, one sign not larger than three feet high by five feet long on a whole frontage of 3,000 feet or more, provided that any such sign is set back not less than 30 feet from any street line and not less than 15 feet from any side line which is not a street line. The top of any sign provided for in this section shall be no more than six feet above the ground or curb level, whichever is higher. No signs shall be illuminated.