8" inline fan. I wouldn't go any smaller. Get one that is adjustable. AC infinity produces fans that will adjust based off the temperature of the canopy if you wanna look into that. BTW I had to google what a roof qube was, just call it a grow tent next time.
If you are ventilating a grow tent, it's important to know how much air to move to provide and adequate amount of fresh air. Since it fills up the entire space, you can guess that the volume of your grow tent is equal to the amount of air that needs to be exchanged. A little more goes into the...