An inline fan has one job: exchange the air in your tent often enough to keep temperature, humidity, and CO2 in range. Undersize it and heat builds under the light; oversize it and you fight the humidifier all day. Sizing takes three steps.
1. Tent volume
Multiply length by width by height in feet. A 4x4x6.5 tent is 104 cubic feet. That number is the amount of air the fan needs to move each minute for a full exchange per minute, which is the usual target for tents with LED or HID lighting.
2. Add for restrictions
Carbon filters, ducting, and bends all cut real airflow. A common rule: add 25 percent for a carbon filter, 10 percent for each 90 degree bend, and 5 percent per 10 feet of ducting. For the 4x4 above with a filter and two bends, that is roughly 104 x 1.45, or about 150 CFM.
3. Pick the fan and control it
Choose a fan rated above your adjusted number, then run it on a speed controller so you can dial it down. An EC fan with a variable controller costs a little more up front and pays it back in noise and power. Pair it with a passive intake sized at least as large as the exhaust so the tent does not pull in like a vacuum.
If you run a dehumidifier or CO2, you will exchange air less often and the fan sizing changes. That is a separate guide.