[2Bad4Ya]

Not a pro, but I would look at the tech sheets of the paint you plan to use. Those sheets tell you tip size, cfm at compressor and gun tip. Example from a House of Kolor tech sheet:
GUN SET UP
• Conventional Gun = 45 to 55 PSI
• HVLP Gun = 10 PSI at the cap
(Refer to spray gun manufacturer's recommendations)
• Needle/Nozzle = 1.3 to 1.5
(Depending on the size of object being painted)
• Trigger Pull = 50% to 75%
• Air Brush: Follow gun manufacturer's recommendations

The paint gun will have requirments also I think. The cfm advertised on air compressors are based on short intermittant(sp?) use, and not extended use I think, but someone may correct me.