I grow in a 4x8 but I only use 6x4 growing area, I have to have a big Evaporative cooler and an AC unit for our heat and low humidity, I would suggest a six-inch for that space, mine is barely adequate for my space after its second season. Bigger is better because it allows for a slower larger volume of air movement and this helps stabilize humidity and keep it around +/-3% of the setpoint, at least with my setup. If you flip that and control the airflow with temperature, likewise it helps keep the numbers closer to optimal. It all depends on your area and your setup. each grow has its own unique challenges.
