Compressor size for HVLP gun?
All most any compressor should be able to handle an automotive sprayer, especially if it's only a few small metal parts. I doubt it'd even kick on half the time. If it does you just have to wait until it stops running (probably not even then).

The (very) important part is the regulator, set to 25psi max, above which the compressor's tank should have no trouble maintaining. If you're trying to paint a car, then you might want to look into a beefier compressor, because you'd want to avoid having to stop midway, to let it catch up.



More "gallons" means longer spans of not needing to run the compressor.

More "horsepower" means less time to catch back up.



Unless you're sand-blasting, really neither of these numbers concern you (at this time that is; go big if you're planing on buying stuff like nail guns in the future). A standard pancake compressor should suffice for just about any task except continuous sand-blasting.