Re: Wire size... ?
Each way has it's advantages and disadvantages.
A friend built a house which remained off the grid for the first five years. He wired the house normally as if for 120 volts, but put 12 volt light bulbs in the fixtures and a bank of batteries in the basement which were connected to a small wind mill and a solar panel. flip the light switch in the wall and the light went on in the ceiling. When he switched to grid power all he had to do was connect the main box to the power company and change to 120 light bulbs. Unless you want separate dual voltage systems, wiring for 120 once makes sense. If the 12 volt is permenent then you want different wire, so folks coming along later can identify the difference and won't tie them into each other.
He also had several 12 volt appliances, coffee maker, electric saucepan, tv etc. he made up a 110 plug to 12 volt socket extension cord to run the appliances until he went with the grid power.