Getting the correctly sized whole house fan is crucial to their ability to cool the home. However, it is less about the actual size of the cooling system and much more about the amount of airflow they push through the home. It does not necessarily take a larger cooling system but if a home is larger then it usually needs a machine that can manage pushing that amount of air.
Quiet Cool has an easy three step rule that explains what fan someone should get in their home. The sizing is determined by CFM, or cubic feet per minute. For a “good” system, you want 2 CFM per square foot of living space. For a “better” system, you want 2.5 CFM per square foot of living space. For the “best” system, you want 3 or more CFM per square foot of living space.
So, if a home is about 500 square feet, the CFM needed for that home to get the best possible system would be a whole house fan that pushes 1,500 CFM while for a home to get just a good system then someone would only need a fan that pushes 1,000 CFM.
Some companies that are selling whole house fans do not abide by this rule, however. They try and compete with great price of other companies by undersizing homeowners, often less than 1 CFM per square foot. This creates a system that is underpowered for the size of the home and the homeowner will be unhappy with their system.
They do this to sell a whole house fan for a reduced price, making it more appealing to a potential buyer. Another important factor is location. If someone lives in the desert, the best system may be the intelligent choice to make. If you happen to reside on the coast then a good system may work well because it usually does not get too hot on the coast.