See the link in my signature below to the sticky on grow room setup. The second entry deals extensively with fan sizing issues. It will help you work through all of your fan issues. Note that a fan speed controller is good to have. You want to over estimate your needs slightly and then then dial it back to the appropriate setting such that you setup doesn't sound like an aerodynamics test chamber. Size it too small and... well you don't get the air exchange needed. Good luck!