It depends on two numbers: your peak AC load, and your daily energy consumption. Work both out before shopping, because a generator is heavy, noisy, expensive to install and needs servicing like any other diesel.
Peak AC load is what decides whether you need one at all. Air conditioning, an electric galley, a watermaker or workshop tools can exceed what an inverter and battery bank will sensibly deliver. Daily energy decides how long it would run — and a generator that runs lightly loaded for hours is a generator that will foul, so it needs sizing to the real load rather than to the largest number in the brochure.
Many cruising boats avoid one entirely with a larger alternator and proper regulator, solar, and a battery bank sized to the actual consumption. That combination is quieter, needs less maintenance and has no exhaust to corrode.
The honest test is to log consumption for a few typical days aboard. Boats fitted with generators that turned out to be unnecessary usually skipped that step.