A fan that is too small will not move enough air, while one that is too large can overwhelm a room.
As a general guide, rooms up to about 75 square feet suit fans around 29 to 36 inches. Rooms of 76 to 144 square feet do well with 36 to 42 inches, and rooms of 144 to 225 square feet with 44 to 50 inches. Larger rooms may need 52 inches or more, or multiple fans.
Ceiling height matters too. Blades should be at least seven feet from the floor; use a downrod for high ceilings to bring the fan to an effective height.
Check the airflow rating, often listed as CFM, to compare performance between models.