The cfm of the oscilating fan doesn't matter, it's only gotta help the exhaust, just evens the temps and prevents any hot spots that might occur without one and provide a little breeze to help with stem strengthening.

My intake is the same size as my exhaust and for most of the time thats enough, however, if it gets very hot i open up another vent that allows more air circulation and drops temps a couple of degrees. So it's better to have your passive intake twice the size of your exhaust for maximum air flow or two intakes the same size as your exhaust, ideally from opposite sides of the cab for more even air circulation.