I have an IR 60g 220v unit, but I have had it for 15 yr, and use it for sand blasting and DA sanders for autobody work. Honestly, the only process that may tax a 20G 1.5hp unit would be the canopy separation. Even using a die grinder with a 20gal would be fine unless you try to overuse the tool for some process.
60G+ 5hp+ is a must if you decide to paint (large panels) and use air sanders. OK, you can do this with smaller units, but the the killer for smaller units is heat build up. If the unit gets hot, then any humidity will condense in the tool and not in the tank. BAD for painting and bodywork. It sounds like that is a ways off for you and is a different decision.
A 20g will handle drills, squeezers, blowers, a trim sprayer for primer on emp skins, and even a venturi vac (for short periods) etc.
Searching will bring up other criteria than air supply, namely noise. If you can isolate the compressor and have it ingest "inside" air, that will handle noise and still take in low humidity air. Suggest you consider a harbor freight automatic water drain device to ensure it stays drained. Especially if you have humid seasons.