Because you can control the speed you're best getting 6 inch. 4 would do, 8 is overkill, 6 is just right, (think goldilocks and the 3 bears) ya need to think of the future too, trust me lol. You may have a bigger space later on. I've got a 6 inch rvk in a 5x3x7 ft groom and I run it on 2/5 on a variac controller. Passive intake hole is just under 1 ft Sq. So passive intake and a 6 inch fan, you're good. Those cloudline are fuckin awesome btw, seen at least 6 ppl buy em elsewhere, good luck mate