How much attic venting do you need?
As you know, when whole house fans pull air into the attic, it must escape out vents. You need to measure and calculate the total attic ventilation area in square feet. Vents include roof mounted openings, holes under the eaves and gable vents usually on the ends of your house. Measure them all and calculate the total square feet.
QuietCool recommends you have at least 1 square foot of venting for every 1500 CFM.
Nutone, a company that makes smaller vents, recommends 1 SF for 1000 CFM.
The state of California has a Title 24 document that suggests 1 SF for every 750 CFM.