Air flow and A paint booth

[JRM]

I know this is a little out of the flowbench relm but air flow is air flow right. Heres what Im trying to build. A paint booth for painting cars. The area I have to work with is 20' x 40' or 800 sqft and I have a 24" fan mounted to the roof, the cfm rating for the fan is 6000 at 1".
What i am trying to figure out is how big of a box do i need to build to be abol to redirect the air flow. Yet not restrict it.