I based it off of Ed Walicki's booth which can be found at:
Ed's Paint Booth
I have the motor up in the attic suspended to reduce noise and vibration. The fan is belt driven and completely separate from the motor which is great for spraying enamels. The down tube is 10 inches and then I branched two 4 inch valved sections down to either side of the booth. I vented it right out the side of the house.
I works great. I can hold a lit match 6-10 inches outside the box and the flame will be getting pulled sideways. I added 5 air connections which come in handy not only for painting but running others tools from that side of the garage.