Hey you would want to look into something called the Bernoulli Principle. Its the law on an inviscid flow, would help you out a lot
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Ideally you want a bit of both. Higher pressure will fire your object faster, so is desirable until you reach the pressure rating of your components. If you don't have enough volume you won't be using the pressure to its fullest extent, but too much volume will just be wasted (and noisier). Storing large volumes of pressurised air is more dangerous than smaller volumes at the same pressure because it stores more energy which would be released if your storage tank fails, so that's a safety concern.
The short answer is you want as high pressure as you can get without risking component failure, and enough volume that increasing the volume doesn't shoot your projectile any faster.
Low pressure is usually quieter.