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.

try one of these bad boys, heard they work great. compact and fast,

Inductor 4 In. In-line Duct Fan - DB204 at The Home Depot

The squirrel cage fan would honestly be your best bet, as they usually move the most air. If it is too loud though, the in-line duct fans tend to work great. Depending on how much smell you need to cover up, you may also need a carbon filter. You should also have a fan for air intake as well, so keep that in mind. Your grow space is very small and I doubt you have more than a couple plants in there, so really any CFM fan will work. The 4" in-line duct fan will move plenty of air, but for your intake i'd suggest a 6" in-line duct fan. You always want more air flowing in, rather then out. Hope this helps.

If you have the funds get a cooled hood.
My last grow the heat killed me in a 2X4 Hydrohut.(2Ft X4ftX almost7ft) The heat was getting up in the 100's.
I ended up wit a cool tube with a 465CFM blower, a 265cfm blower for exhaust, a 4" inline for intake, and a 6" osculating fan and 2 4" fans inside for air movement. Now my temps stay around 74-85 but today got to 87...
I am still shocked at how much heat a 400w puts out, and think my cool tube was the best investment i have made, and now i can drop the light to about 12" without worring... It sounds like alot of money but i got the 465cfm blower for $99 delivered and the 265cfm for $79 off of ebay, but its worth it since i figured i lost almost $200 in quality seeds...