A smaller compressor will work where a larger one won't...and vise versa. It all depends upon how you pipe the air flow after it gets to the tank and before it gets to the plasma cutter. Air tank size isn't the limiting factor here, moisture is. An on-board compressor leaves the least possibilities for dry air since the tank, if there is one, is small, piping is severely limited and there isn't much room in the box for coalescing filters/moisture filters and such.
I would be more concerned about the air lines and filters after the compressor than the size of the air tank. Install as long a run of air line as possible prior to your first moisture/coalescing filter so that the air from the compressor's heat cycle has a chance to cool and allow the air to "shed" its bonded moisture. This can be done as simply as running the air line up to the ceiling and down to the floor a number of times...or across the back wall. A tank mounted "spitter" isn't a bad idea as well. Finally, a good quality moisture filter at the back of the plasma cutter should have you up and running with long consumable life if you've installed all the other "fixes".
Miller 251, Lincoln PrecisionTig 275, Miller DialArc 250 AC/DC, Hypertherm 900, Bridgeport J-head, Jet 14" lathe, South Bend 9" lathe, Hossfeld bender with a collection of dies driving me to the poorhouse, Logan shaper, Ellis 3000 bandsaw, Royersford drill press and a Victor Journeyman O/A.