For my 4x4 tent - I use a 6inch exhaust fan about 440cfm (currently running at 70% more or less) and also a 4inch intake a about 200cfm (100%) - you can use passive intake and should work well too
i also have 3x6inch clip fan(in corners of the tent) for airflow over the canopy and I have a 10 inch fan under the canopy. I also use 2 clip fan to cool down my LED heatsink..
Sometime I think its a bit too much.. but.. good air flow is key to avoid many problem in your tent.. you can look at my grow journal for pictures..
Finally I know its a little off topic but just something to consider for your build.. with my 4x4 and negative air pressure in the tent, the walls were really pushing too much on the inside making me loose a lot of grow space - I was loosing about 6inches on each side so I built up a cage so now I can benefit from all my space- the pipes did provide me with something to add my scrogging net and attach anything else if I needed. i used white pvc pipe.