Old Red is so right, the size of the tank makes no difference. It takes x minutes to fill it, once it is drained down to the kick on point of the compressor it is going to run continuous until it either gets the pressure back up to shut off, or run until you quit using air, then run x additional minutes until the tank is back to full pressure.
If the tool says it used 9 cfm @ 90 psi and your compressor will only make 6cfm @ 90 it will never keep up no matter how big the tank within reason. Maybe a 5000 gallon tank would last long enough for you to do something before the compressor had to run, but it would run a loooong time to fill and refill the huge tank.
The bottom line, when using air sanders and spray guns, the usage is quite substantial and as such takes a substantial air compressor to supply air.
20 amp, 240volt motor is about the smallest that can keep up. Less than that you can get by, by doing panels and waiting and dealing with moisture issues.