Electric wiring procedure starts from the future and works to the present. You must first know how many outlets and appliances you plan to serve. Then the wire is selected to carry the amperage to the addition. The wire can be larger than necessary to prevent fires. Electrical code practice often requires the breaker to be no larger than 1-1/2 times the amps carried through the wire (to prevent fires).
Your local code officials are your friend in this work; they have to approve the work for reasons of safety and they know the wire size. The fire department will investigate the wire size and breaker after a fire, as well as for insurance coverage questions.