Long ago I had good success with cheap Home Depot guns and a 20-gallon 2hp compressor like Nessism mentioned. Today I use a 60-gallon 5-hp compressor, it states that at 40lbs of pressure it puts out 13.2CFM's, the guns I use are mini-guns so they take anywhere from 4-6 CFMs. This combo has worked quite well for me. I believe the general rule, though don't quote me, is to take the maximum CFM of your tool and multiply it by 1.5X to determine the minimum CFMs you need to provide it. I can say that when I was using the 20-gallon compressor it would turn on quite frequently so theoretically at those moments my spray gun was underpowered. Unless you have the gun already, you don't need a full size automobile spray gun to paint bike frames, they tend to be air hogs.