When choosing the size and filter for your grow room, you will need to calculate its cubic footage. This may be accomplished by multiplying the length by its breadth and height.In an ideal case, all of your air should be replaced every minute.
Therefore, your ventilation system including the carbon filter and fan shall be able to displace the required air volume in the stipulated time. You can calculate the air flow (CFM) requirement as follow and then select the suitable filter that can cater for this flow requirement.
Measure your tent length, width, and height in feet
Multiply the three dimensions to get the tent volume. Volume= Length x Width x Height
Three times the tent volume will be the CFM requirement of your Grow Tent, and you can select the filter as per the CFM requirement.