What size ceiling fan for my room?
Fan size is the blade span — tip to tip across the circle. The usual guide: up to 75 sq ft takes a 29"–36" fan, 76–144 sq ft a 36"–42" fan, 145–225 sq ft a 44"–50" fan, and 226–400 sq ft a 50"–54" fan. Over 400 sq ft, go to 60" or larger, or use two fans. A 12' x 15' bedroom (180 sq ft) is a good fit for a 50"–52" fan, the most popular size.
One fan or two?
In a long room — more than about 18 ft — one fan in the middle leaves the ends still. Two medium fans, each centered in half of the room, move air more evenly. The planner splits rooms like that and sizes each fan for its half.
How high should a ceiling fan hang?
The blades must be at least 7 ft above the floor. ENERGY STAR recommends 8–9 ft when the ceiling allows, and at least 18" between the blade tips and the walls. On an 8 ft ceiling, a flush-mount (hugger) fan is usually the only way to keep 7 ft of clearance.
Downrod length
Subtract 8 ft and the fan's body height from the ceiling height, then pick the stock downrod at or just under that number. With a 12" body, a 10 ft ceiling takes a 12" downrod and a 12 ft ceiling a 36" downrod. Fans usually include a short 3"–6" rod for 9 ft ceilings. On a vaulted ceiling, use a slope adapter and check the maker's chart for the minimum rod length at your pitch so the blades clear the roof.
Mounting
A ceiling fan needs an electrical box rated for fans, fastened to a joist or a fan brace. A standard light-fixture box can't carry the weight and wobble.
Sources: ENERGY STAR — ceiling fan basics. Cooling a room? Size it with the AC size calculator.