I would suggest a 12 ga.. But the more common 14 should be OK in open air.
Still.. I always recommend the heavier cord and Sears (And K-mart) both sell a very nice 12GA cord with a LOCKING outlet.. By this I mean there is a button on the outlet, you push the button before you plug into the cord or unplug,, I really like those cords and have 100' of it in my basement just now.
Also, if you have a 2nd Air conditioner... (or add one) I run my 2nd A/C using a shorter 12GA cord when I'm on a 30 amp site... Off the park's 20 amp outlet.