As a general rule of thumb, (for passive intakes) they should be 2x the exhaust. So if using two 120mm fans' you'd want four 4" intakes. Stay away from bends or ducting on any kind. ...more drag/resistance is created n' not worth the hassle. I've been contemplating how to do this setup on my suburban for over a year now....best I can come up with to maximize airflow is two 120mm fans in each rear passenger window exhausting while pulling in from the rear barn doors.