The size of the compressor tank is a secondary and minor factor when spray painting. You need to look at the performance ratings of the compressor itself.
Typically a compressor will be rated in terms of SCFM at a certain pressure. Those numbers need to be greater than the air required for whatever tool or spray gun you are using.
If your compressor operates on 120 volts the largest it will be is 2 horsepower. That is very marginal for continuous spray painting of a large area. A very large tank does increase the time before you need to stop painting.
