Resistance literally means the obstruction to the path of flowing electrons in the form of current due to the properties of a metal. So with this background if we see, the longer the rod ot the object through which current or electrons are flowing, the more will the electrons collide or hit against the molecules of the metal, as the path these electrons need to travel is now longer. As a result of this logic, it is observed that as the length of a metal rod or object increases, the resistance to the flow of current also increases. Therefore, resistance is directly proportional to the length.