How to size a whole house fan?
When I sized the ventilation (with heat recovery) for the house we built, I used numbers based on the replacement rates for the activities in each room combined with the number of people.

So, bedrooms tend to have the lower requirements, kitchens etc highest.

We had extraction in kitchen / bathroom and input in bedroom / living areas etc  The unit we fitted, that was designed to run 24/7, was a unit 2ft * 2ft * 1ft deep. We also took the opportunity to bring the inlet air underground for 100 feet to help mitigate the summer high and winter low temperatures of the air.

We used this supplier : https://www.stiebel-eltron.com/en/home/products-solutions/information-engineering/ventilation-systems.html

Have a look at this link as that will give some relevant values:

Room air changes

Once you have the total amount of air to move, then you can work out the amount per hour and from that the fan size needed.