Measure your room's width and length in feet. Multiply the width by the length. This is your room's square footage.
Choose a fan with 26- to 36-inch blades for a room up to 75 square feet.
Choose a fan with 36- to 42-inch blades for a room from 76- to 144-square feet.
Choose a fan with 44-inch blades for a room from 144- to 225-square feet.
Choose a fan with 50- to 54-inch blades for a room over 225-square feet. Apply the ratios from there, or buy multiple fans using the same ratios. For instance, buy 2 fans with 44-inch blades for a 450-square foot room.
Choose a fan with standard mounts for typical 8-foot ceilings.
Choose a fan with extended mounts for 9- to 20-foot ceilings.
Choose a fan with flush mounts for ceilings under 8 feet.