Proper fan size is important to ensure proper air flow.  This chart should help you determine what size fans you need for your home.  Beyond fan size, you should also keep in consideration fan height.  Recommended fan height should be 7 to 9 feet above the floor depending on your ceiling height.  You never want the fan to be a hazard for taller occupants and at the same time, a fan that is too high on high ceilings won't properly move the air low enough for you to feel a difference.  Another fact you need to consider is if your ceiling is sloped where you need to install a fan, you need a fan that can be mounted on a slope canopy.  Always verify that there is enough clearance for the fan blades in any application.  We are always happy to help you pick the right fans, down rods, light kits, and accessories for your home.