When choosing ventilation for your grow tent, the important consideration is how much air your plants need. First, divide the circumference of the room you're growing in by 3 to get the cubic feet of area that needs ventilation. Then measure the required opening size for your fan and multiply it by the number of CFM your fan can create. Finally, add 10% extra for flow loss through a cotton filter.
For example, if you have a grow tent that's 12x12 ft and requires 200 CFM of ventilation, your fan will require a 24 CFM rating.