The answer to 'what size circuit breaker do I need' is that the size of the circuit breaker you need depends on the electrical load and wiring of the circuit. As a general rule, the circuit breaker should be sized to match the capacity of the wiring it is protecting. This means choosing a breaker with a current rating that is less than or equal to the maximum current-carrying capacity of the wires.
For example, if you have a circuit with 14-gauge wiring, the maximum current-carrying capacity is 15 amps, so you would need a 15-amp circuit breaker. It's important not to oversize the breaker, as this could create a fire hazard. If you are unsure of the proper size, it's best to consult a licensed electrician who can assess your specific electrical system and make the appropriate recommendation.