Drop rods are often called extension or down rods. They are important as they bring the ceiling fan down to the correct height for the correct and most efficient operation which is about a meter above head height. The length of the drop rod will be determined by the ceiling height and the drop of the fan. If your ceiling is above 2.8m/9.2 ft then the length of drop rod that needs to be added will be whatever is needed to keep the fan at a 2.8m/9.2 ft height. If the fan blades are properly balanced then there should be no wobble even with a long drop rod.
Hunter has designed a patented trilobular mounting system to ensure a perfect wobble-free operation even for very long drop rods. Hunter drop rods also incorporate proper locking pins to ensure safe lifelong operation. The wires carrying power and control signals to the fan are threaded within the drop rod so beware that they will need extending if you use a drop rod longer than 24″.
Note that in certain cases a ceiling fan will require a thread at both ends of the drop rod. This will be specified in the fan description.