What size breaker should be used for a circuit?

A breaker is sized indirectly by the load current. The breaker is used to protect the wire from being overloaded by amperage. Wire is sized by the load amperage. The procedure is, use the load amperage to find the correct wire size that will carry that amperage safely. Size the breaker so that the amperage on the wire doesn't exceed the rated capacity of the sized wire.
The only time a breaker is over sized above the wire's rating is when a motor is connected into the circuit. Then the breaker is over sized by 250% of the motors full load amperage. This is to compensate for the starting current of the motor which can get as high as 300% above the motor's run current.