Hi,
First, DO NOT use a cardboard box as your booth. If you're going to build one, do it right. A good fire-resistant plywood or sheet metal are the best materials.
As far as CFM goes, it is predicated on your booth size. In order to maintain adequate capture, face velocities should be between 80-100 fpm. So, if your booth face size is going to be--let's say--2ft. X 2ft., then the area = 4 square feet. Multiply this by the required face velocity and you get a MINIMUM cfm of 320-400 cfm. Depending on the length of exhaust ducting, you may need to bump up your cfm by about 20% or so to ensure adequate duct transport velocities. Your exhaust ducting should be constructed of at least 4" aluminum flex duct. Stay away from plastic dryer hose. In addition exhaust ducting runs should be as short and horizontal (downward is even better) as possible, and with as few bends as you can get away with. Try to avoid right angles. With every bend and turn you add airflow resistance.
Your fan should be of the shaded pole type. A good squirrel cage fan is excellent for this type of application. The motor should not be in the direct airflow of the vapor stream. For this reason, stay away from range hoods, bathroom fans, computer fans, and the like.
I would ecourage you to do a search on spray booths in this forum. This subject has been covered fairly extensively, and the information could help you immensely. If you have any questions, please feel free to shoot me an e-mail. Other forum members will also provide a wealth of information on this thread. Good luck in your build.
Gip Winecoff