Not all down rods are created equally and most down rods are manufacturer and model specific. Some fans require special down rods, like the Minka Aire Pancake and the Casablanca 19th Century. Most fans will come with at least one, sometimes two, small downrods that will enable a fan to be mounted onto an 8 foot ceiling. Accessory down rods come in the following sizes: 12, 18, 24, 36, 48, 60 and 72 inches.
If you need a longer down rod for exceptionally tall ceilings, come companies make down rod couplers that will enable you to drop the fan lower than 6 feet.