Originally Posted by k_buz
I don't agree with this answer. The wiring isn't dependent on the disconnect. The wire size is dependent on what the AC unit calls for. Many units today only require a 20A 240V circuit. In this case, you would run 12/2.
If the unit says something like Min Circuit Ampacity = anything under 20A there is a good chance you can run 14's even.
This answer is only accurate for the NEC. If you can post a picture of the information provided on the plate, we can tell you for sure.
How can you disagree with me when I said the same thing, I never mentioned anything about the disconnect in my answer.
To the OP look at the name plate for 3 important numbers
Voltage(most likely 240V)
MCA(minimum circuit capacity-you can size your wire to this)
MOPD(maximum over current protection device-size your breaker to this)