Actually I think the size of his home made booth is just about right.
Really depends on what you build. If all you build are 1/25 cars, or 1/35 armor, then yeah, something 2'X2'X2' is probably fine, but is someone is a large scale building, ship builder, or aircraft builder, you need something quite a bit bigger.
I have one I built that is centered around a 16" X 24" furnace filter, and is likely 24" high, which is good for most jobs, but for something like a 1/48 B-17 or Lancaster, or 1/72 Vulcan, B1B, etc, etc. I would have to be extremely careful moving the kit around inside the booth as to not bump it off the sides of the booth.
Ian, if it works for you....have at it my friend.
As far as the motor in the fan being an open winding motor and suspect for setting fumes on fire, the one in my booth is a 100cfm bathroom fan, open motor, and I have sprayed enamel, lacquer and acrylics in there with no problems at all. 99% of the time, the thing with people blowing themselves up with that type of motor in their booth is they are using aerosol cans of paint, and it is the propellent in the can that is igniting. The propellent for the most part is much more combustible than the paint is.
Anyone wants to see my home made paint booth, follow the link:
http://public.fotki.com/ScottM/models/paint-booth/