18 to 20 AWG should be sufficient for each zone wire. You are correct that in your case the common wire needs to be thick, because if you plan to run several vales at the same time, the entire amount of current has to run through the common wire, but the return path is through each zone’s corresponding zone wire, so the common wire needs to be quite thick. I believe 2.5mm^2 is sufficient for common. You can always use parallel connection to combine several thin wires to achieve the effect of a thicker wire.