Ventilation!!!! Need Help

hi my grow space is 2ft x3ftx6ft high. i have a 400 w hps light it gets hot in there so i been leaving the door open. i wanna get a fan to vent it through the plywood into the attic. any suggestions on what size or cfm to use? please help im flowering soon and i dont want to have to keep the door open obviously. any info would be great thanks! i have a squirrel cage fan in my attic but im afraid it might be to big and noisy. didnt try it yet. it runs 6 inch pipe.

Hi mate, I have a 600hps in similar space. I had a 6" and it wasn't enough so I bought an 8" & an odorsok(which restricts flow to 6: standard) and it just does the job with 2 small fans also, 1 over tops and 1 up at light towards exhaust.
You should have a while till she stinks, mine took say 3 weeks to start smelling enough to need odorsok to control it. Any pics?