Not more than one commercial vehicle shall be parked on a lot used for residential purposes. Such vehicle shall be owned, leased or regularly operated by a resident of the premises, or if not the resident, then on property that is at least six acres in size, and said vehicle shall be limited to vehicles which are no more than 26 feet in length and have a GVW rating of 17,500, or is a school bus, and this provision shall not be construed to limit the number and type of commercial vehicles in use on a farm and in connection with farming operations conducted on the property.