Minecraft wiki entry for Building_a_metropolis, associated text: Small downtown street:
Recommended material: Stone, obsidian, or any other road-like material.
Keep it as straight as possible, even if terraforming is needed.
Keep it as flat as possible, even if terraforming is needed, it may be small, but remember, thousands of people, rickshaws, pigs, horses, cars, chariots, carriages, magic carpets, elephants or pod racers use it daily.
Should generally be the equivalent of 1-2 lanes with 2 sidewalks, this could be anywhere from 5-17 blocks wide if sidewalks are included.