Line voltage is the most important factor in determining the size of a line conductor.

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Line voltage directly determines how much current must flow to deliver a given amount of power. Since current is the amount of electricity the conductor must carry, and heating in a conductor is proportional to the square of that current (I^2R), higher current means more heat and a larger conductor is needed to stay within temperature limits. For the same load, increasing the line voltage reduces the current required, which reduces heating and allows a smaller conductor size. That direct relationship between voltage and the current the line must carry is why line voltage is the most important factor in sizing a line conductor. The color of insulation, maintenance frequency, and weather conditions can influence other design considerations, but they do not determine the basic sizing the way the current drawn by the load does.