On the power side I might use 14 or 16 gauge wire. On the load side, depending on the current draw of the individual circuits will determine the wire size.
The headlight, ignition and charging system are the big draw items. I used 16 gauge wire to them.
My turn signals and tail/brake lights are LED's, 18 gauge is plenty. using the stock signals I might run 16 gauge.
The horn, depending on it's current draw, stock don't draw as much as louder after market horns. 16 gauge there should work with either. A relay can be wired in to carry the current without as much running through the push button.