It's not hard. You size the breaker to provide over current protection for the conductors used.
With a few exceptions.
One of them is hermetic motor-compressors. Like your air conditioner. With those, the conductors are sized, by the manufacturing engineers, for the proper ampacity; and the breakers are sized, again by the engineers, to avoid nuisance tripping at it's design conditions. In theory, you could work out the math (per NEC section whatever) and get the same answer.
You are allowed, of course, to go larger on your wire size. The Code is, after all, a minimum that should guarantee a safe install. But you don't HAVE to, unless you want to spend the extra money on oversized wires. Which is fine, but sometime Sparky gets a little worked up about it.