It is important to remember that the first function of a spray booth is to remove the overspray to prevent the formation of a combustible or explosive environment. You should really be doing a minimum of two air exchanges per minute. Thus, if your booth is 9x20x8, or 1440 cubic feet, you need to move at least 2880 CFM. Box fans are often used in homemade booths but should be considered inadequate, as they are intended to operate in a 0 static pressure environment. Curved blade fans, or squirrel cages, offer the best alternative as they can be operated over a large range of speeds and static pressures.
The key to designing a good homemade booth is to determine the airflow you require, as described above, and then build the fan and filter system to produce that flow. Filters, even light ones, greatly increase static pressure, or restrict air flow. When static pressure rises, air volume decreases. As air volume (hence mass) decreases the load on the fan motor decreases. This effect will allow you, with the help of an ammeter, to roughly determine the amount of air your fan and filter system is moving. Broadly, in the range of static pressures typically found in homemade booths, 1 horsepower will move about 1800 CFM. If you need to move3600 CFM, then a two horsepower motor should suffice. Use your ammeter to check the actual amp draw of the motor, and compare that to the Full Load Amp rating of the motor (usually listed on the motor id tag as FLA) If the motor is drawing its full load amps, then it is producing full horsepower. If the amps are low, put a larger sheave (or pulley) on the motor. Be sure to test the motors only with all filters in place, and the booth running as though you were painting.
You will need two different filter areas for your booth. First is the intake filter area. These filters provide clean air inside the booth. If you are using furnace filters, you should have 1500 square inches of filter per 1000 CFM minimum. The second set of filters are the paint arrestors. Use fiberglass filters if at all possible, as they last longer are less likely to spontaneously ignite. Use 1000 square inches per 1000 CFM. You can never have too much filter area, by the way.
If possible, use a rigid sided booth, say drywall as opposed to plastic, unless you build a positive pressure booth. The plastic creates turbulence as it flaps and shakes with the air. For the highest quality paint job, you want to maintain laminar flow throughout the spray area.
One final thought. Positive pressure booths, which have an intake fan to force air in, and then an exhaust fan to pull a little less air out are the best booths. You must balance the fans to produce a positive pressure of no more than .3 in. w.c., but that is easily done with dampers, or with the size sheaves you put on the motors. Positive pressure lessens the need to get all psychotic about sealing your doors, as you can not suck trash in from your shop. (Your a G.I., think of NBC clean rooms, or clearing your gas mask)
Hope I helped. Feel free to ask for clarification