AC UNIT Breaker size
The breaker is sized for the protection of the wire and device. The disconnect must be sized larger than the current flowing through so that it doesn't burn up, but is just a switch.

If the AC unit says that it's breaker should be a max of 20A, then use a 20A breaker.

However, just to be clear, you can't change out the breaker to match the unit if the wire sizes are still too small. Since it says to use a 20A, the wires should be 12 gauge or larger. Any smaller and they could burn up before the breaker would trip.