Bathroom fans are rated per the airflow in CFM (cubic feet per minute). The rule of thumb is that you need at least 1 CFM per square foot of room area, if your bathroom is less than 100 square feet. So, a bathroom of 60 square feet would need a 60 CFM fan. For bathrooms larger than 100 square feet, add up the CFMs associated with each fixture you have to determine what size fan to buy.