If you have (or use) only 1 A/C unit, then you may get by with 3KW, which is not very small, and 4KW is better. If you have 2 A/C (and want to use them at the same time), you'll want to be in the 6KW to 8KW range.
For size, 2 Honda EU2000 hooked together is hard to beat. Also, true sine wave power and very quiet and pretty efficient.
bsadams - With a 7200 watt contractor trailer I run 2 air conditioners, all lights, fans, etc. I am not recommending my generator but just trying to let you know you will need at least a 6kw to run what I run. I am planning on getting a Kipor 6000 watt for my purposes. Runs about 2k dollars and is extremely quiet, enough for any camp ground. :bleh: Jim