Look for the cfm (i.e. the air consumption) of the tool you'll use for the longest duration, then at least match it.
E.g. let's say you've got a DA sander needing 6 cfm, and a wheel gun needing 10 cfm - you size the compressor for the 6, as that's needing a sustained flow of air. The nut gun only needs short bursts, the tank will handle it alone.
The other spanner to throw in: how much electric power have you got available to run the thing?
If you can only plug into a 13A socket then you're limited to about 3hp compressors off-the-shelf.
Another option is to buy just an extra cylinder, and connectit it with the existing compressor...
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