It depends on a lot of things -- how many people, how many days, whether you have LED lights, what kind of battery. If you want to run the a/c, the smallest would be 3000 watts (or two 2000s hooked together). It'll be noisy, though. Try to get Honda or Yamaha -- much quieter than the contractor-grade generators. If you are in a campground running a loud generator, you will not be popular with the neighbors.
We camp mostly at high altitude and in cool weather, so we almost never use our a/c (except while staying at RV parks with hookups, when we are traveling, as distinguished from camping). So we almost never run our generator -- we have a Honda 2000. It is quiet but not silent. I like silence.
We recently got a 120 watt portable solar panel -- more than enough power for two people. But we are pretty frugal with electricity.