Following are the concise measurements, though a lot may change according to your room size-
For small rooms up to 64 square feet, get a 29" fan.
For rooms up to 100 square feet, a 42" fan.
For rooms up to 225 square feet, a 52" fan.
And for large rooms up to 400 square feet, one 56", one 60" or two 52" fans will be best.
Place the fan as close to the center of the room as possible. If you have an irregular space (like an L-shaped room), consider mentally splitting the room and hanging two fans sized for each space.
The blades of your fan should be at least 18 inches from the nearest walls.
The optimal height from the floor for your fan depends on the height of your ceiling. After you've figured out the height of your room, check out this graphic, from Garbe's, to find the optimal down rod length (the bar that travels from the fan's base to your ceiling) to hang your fan:
Fans should never be hung lower than 7 feet from the ground. If your room is shorter than 8 feet, consider a low-profile venting fan that's designed to hang flush on the ceiling.