I'm currently using a 30 gallon/6hp compressor. It will keep up with my Iwata with no problem. With the Finishline lll, it will run a bit more. Using an air hog, like a HF purple - might have some real pain in air flow. The major consideration that many don't look at until it's too late is 'Duty Cycle'. If it's 100%, (60 minutes running/hour) you probably are golden. 50%(30 minutes), it's not so good. Less then 50% - don't bother bringing it home as it's only good to fill tires and maybe run a primer gun while doing a panel or two.
And those Hp numbers - they are 'developed' horsepower. That 6Hp on my compressor - might be at inrush current, first startup, but returns to running Hp once it's in operation 1-3 or so. My little portable Sears 1.5Hp was just that and had a 100% duty cycle - but it died
wouldn't run much of anything but a siphon feed gun and fill tires - kinda)