Pick the right ceiling fan blade span for a room size.
Ceiling fan sizing is a floor-area lookup, because what you are matching is the diameter of the air column the blades throw against the size of the room they have to stir. The industry bands run roughly 36 in up to 75 sq ft, 44 in to 144 sq ft, 52 in to 225 sq ft and 60 in to 400 sq ft; beyond that a single fan leaves dead corners and two smaller fans move air better than one huge one. Ceiling height sets the downrod, not the span: blades should hang around 8 to 9 ft above the floor, which means a flush or 3 in mount at 8 ft and roughly 6 in of rod for every extra foot of ceiling. Hanging a fan too close to the ceiling starves the intake and cuts airflow noticeably even though the blades are the right size.
Ceiling Fan Size
Blade span from floor area bands (75/144/225/400 sq ft); downrod = (ceiling height - 8.5 ft) x 12 - 12 in, rounded to a stock length
Between 8 and 9 ft, and never less than 7 ft. That band is where the airflow reaches occupants without the fan feeling oppressive or being a head-height hazard.
Two, once the room passes about 400 sq ft or is markedly rectangular. A single 72 in fan moves plenty of air directly beneath it but leaves the ends of a long room still.
Span sets the reach, pitch and motor set the volume. A 52 in fan with a 12-14 degree pitch and a DC motor will out-perform a 60 in fan with flat blades, but it still cannot cover a larger room than its sweep.