If you are unsure then go big. Bigger the better because you can always get a speed controller and turn the fan down. Bigger fans running at lower RPMs are much quieter then a smaller fan on full. Plus if you plan on using a filter or an air cooled hood you will need more CFMs.

Also you want your exhaust with the filter attached up high as possible. That way its removing the hot air that is rising and more efficiently cooling the cab as well. The smell tends to rise on the hot air also.