No parcel of land shall be less than thirty-five (35) acres in size if in the A-1 zone, or less than five (5) acres if in the A-2 zone, nor shall any parcel of land existing in single ownership at the time of passage of this Code henceforth be divided for sale in units of less than thirty-five (35) acres if in the A-1 zone, nor less than five (5) acres if in the A-2 zone.