Fans used along with air conditioning save energy by allowing you to raise the temperature on your thermostat without feeling significantly warmer. In most cases, people are able to raise their thermostat three to four degrees and feel just as comfortable as they do without the fan. Why? It's because the movement of air cause by the fan evaporates moisture on the skin, making you feel cooler. And that higher thermostat setting translates to lower energy costs, usually around 25 percent. In fact, every degree your thermostat is raised during the summer can save 7 percent to 10 percent on your electric bill. Keep in mind, though, that you don't save any money unless you actually raise the thermostat.

Ceiling fans should only be installed in rooms where the ceiling is at least eight feet high. Fans function most efficiently when the blades are 7–9 feet above the floor and 10–12 inches below the ceiling. To be safe, the fan blades should be no closer than 8 inches from the ceiling and 18 inches from the walls.

In order to achieve the ideal Height, a number of different mounts are available. The standard mount includes a 3-5 inch downrod, the metal pipe that connects the top of the fan to the ceiling bracket. For high ceilings, you can purchase a longer downrod--anywhere from six inches (for a 9-foot ceiling) to 120 inches (for a 20-foot ceiling). For ceilings that are a little low, consider a fan that mounts flush with the ceiling with no downrod. Finally, most fans can be used with angled ceilings, up to about 34°.

If you want to put a ceiling fan in an area with lots of paper or other objects that can be easily blown around, consider a fan with large blades. Large blades provide a greater cooling effect at lower speeds than small blades. It's also important to choose a fan appropriate to the size of the room. A large fan in a small room will appear out of place and can make the room feel even smaller.

Fan blades range from 29 to 72 inches in length. The most popular size is 52 inches. Selecting the right size depends on the size of the room. The following chart will help you decide.

Room	Blade Size	Max Sq. Ft.
Great Room / Living Room	60"+	625
Master Bedroom / Den / Living Room	50"-56"	485
Bedroom / Office / Kitchen	42"-48"	225
Laundry Room / Bathroom	32"-36"	100

Source: http://www.hunterfan.com/pages/how_to_select.php