Ceiling fan blade length, sometimes called blade span, is often given in inches or cm. For example, a 56" fan would be the total width across the whole fan.
All other things being equal, longer blades provide more airflow at slower speed, usually making them more energy efficient and quieter since they require less revolutions (turns) per minute for the same air movement. Shorter blades create a more concentrated flow of air, suited for smaller rooms and targeted areas.
We recommend looking at the dimensions of your room and reviewing the air movement of the fan model you're considering.