Check our CFM calculator to find the required airflow in cubic feet per minute (CFM) for any room size, depending on its recommended air change per hour.
www.omnicalculator.com
From personal experience, I'd go with a 6" exhaust My AC infinity 5x5, 3 bucket DWC grow requires my 4" AC infinity exhaust fan to run 24 hours a day on speed 10. My tent setup is in a room in my basement so once winter hit I was good on keeping tent temperature down. I run a 600w HID as well. Was seeing 79-82°. It couldn't keep up with maintaing a lower RH when switching to Flower, before introducing a dehumidifier. A 6" exhaust would of solved a bunch of my issues.
Like
@Lenin1917 said, the outdoor environment the tent is in, is of equal importance! My dehumidifier as well as my exhaust for the inline are all vented out of the room.
https://www.rollitup.org/t/cheap-diy-high-cfm-bars-10-5-x5-tent. For when your exhaust is sucking the sides in. (Could always open a vent like I do but a controlled inside environment is ideal for quality herb)