Id say the more air the better for sure..
It all depends on who you talk to, you will get several different opinions for sure..
Some I have seen run 8 inch inline in 4x4 tents and say it's not enough, others run a 4in in that same space and say it's enough..
Myself I have an 8in inline with no carbon attached "no need for it, I live on 200+ acre plot" and I wish I had a 10-12 inch, but the 8inch was there and never used, so Im making due with it..
Some will tell you that exchanging the air 5 times in 1 minute is the goal to reach "which I agree, the more air makes me feel better", but some have also said that you want to exchange the air 1 x every 3 mins..
It will vary depending on who you talk to, but Id say get as much air going as you can afford and safely add to it "without drawing attention", these inline fans get loud as fek.