How to choose a downrod for your ceiling fan.

Ceiling fans should be installed, or mounted, in the middle of the room and at least 7 feet above the floor and 18 inches from the walls.  If the ceiling height allows, install the fan 8 - 9 feet above the floor for optimal airflow.

Standard mounts come with a 3 - 6 inch downrod, which is the metal pipe that extends from the ceiling bracket to the top of the fan.  Downrods are usually either 1/2 or 3/4-inch in diameter.

Extended mounts can be used to hand the ceiling fan at the desired 8 - 9 feel from the floor for tall ceiling heights.  Downrods can be purchased from 6 inches (for 9 ft ceiling) to 120 inches (for 20 ft ceilings).  

Flush mounts anchor the fan directly against the ceiling.  Most standard fans can be installed as a flush mount however, many manufacturers sell "hugger" or "low profile" ceiling fans that are specifically designed for this purpose.  Hugger fans are ideal for rooms that have ceilings under 8 feet in height or in situations where a fan with optional light would hang down too low.  Hugger fans will not move as much air as a regular fan because the blades are closer to the ceiling.

Sloped mounts are used for angled or vaulted ceilings.

Source: Energystar.gov