Jeremy,
I did a quite a bit of research and thinking before selecting a compressor for my RV-10 build. Like most things in life and building there are trade offs and so no single right answer.
When it comes to compressor size there are 2 key measures of size. The first is the compressor (air pump) output, measured in standard cubic feet per minute (cfm) at working pressure. Typically 90 psi pressure is used for rating air compressors for our use. If you have a compressor with an output that is equal or a bit higher than the air volume used by your tool, your air tank can be very small without impacting your work.
The second key measure however is tank size. The tank stores the air and allows the compressor to cycle on and off rather than run continuously. Since most tools we use only draw air for short periods of time (from a few seconds for a rivet puller or squeezer to a few minutes for an air drill), a large tank can provide the air at higher volume than the compressor can produce for a period of time, allowing a smaller compressor to support your work. But when you require a high volume of air for an extended time period as required for a spray paint gun, the stored air in the tank will run out before you have completed the task, requiring you to stop and wait for the compressor to catch up by refilling the tank.
So you are looking for a balance between compressor size and tank size. But compressor price increases with larger capacity, and the power required increases driving a change to higher voltages and amps, and then to 3 phase power in industrial size compressors. And as tanks get larger, weight goes up, portability goes down, and price goes up. Higher operating/storage pressure is another factor that can help storage volumes but also drives up cost, weight, power, etc.
Another factor worth considering is noise. Belt driven, lower speed, oil lubricated compressors tend to be less noisy and longer life than direct drive non-lubricated compressors, but these also cost more. So unless you can place the compressor in a different location or sound attenuating enclosure, noise (measured in dB where a higher dB is a louder machine, and every 3 dB increase doubles the noise level) is also a selection consideration.
So what is the electrical capacity in your shop, how much space do you have for the compressor, how much noise can you accept, how much money do you want to spend, do you want to be able to move the compressor without a crane or pallet jack, do you want to do high volume painting, etc. are a few of the considerations.
For my shop, after weighing all these considerations including reliability, longevity, availability (what compressors are available for purchase), etc, I ended up buying a Quincy 26 gallon, 7.4 SCFM at 90 psi, portable electric vertical compressor. This was the highest capacity I found that would run on a 120 VAC, 20 amp circuit, had wheels and so could be moved without tools, could be re-wired for 220 VAC in the future if I wanted to do so, was oil lubed, belt drive and so relatively quiet, assembled in the USA with a good reputation for reliability and reasonable quality.
I bought mine from Lowes
https://www.lowes.com/pd/Quincy-Com...ble-Electric-Vertical-Air-Compressor/50073349 as they were in stock and lower price when I bought mine a couple of years ago.
I have been very happy so far with my selected compressor. I have been able to do some painting, including primer and painted the interior of the cabin. I plan to have the exterior professionally painted, so don?t need a larger compressor and paint booth. I only moved it around my shop a couple of times, preferring to leave it in one place next to a power plug and using a long hose instead. I was pleasantly surprised to find that it worked well on a 120 VAC, 20 amp circuit that also has some lights on it, so I didn?t need to run a separate circuit or convert it to 220 Volts. So for my purposes, I would give it a full 5 star rating.
Sorry for the long dissertation, but I find that a lot of people don?t fully understand the air compressor sizing and compressed air physics that impact the selection and satisfaction, and often just look at the air tank capacity. Hope something in this post is helpful.