Minecraft wiki entry for Historical_effects, associated text: Redstone dust dot past X/Z 1,073,741,824 (halfway to X/Z 2,147,483,648), at a spot where it would render, on only one axis.
Giant redstone dust past X/Z 1,073,741,824 on both axes. It appears 128 blocks wide in both directions.
A superflat world with redstone past X/Z 1,073,741,824 blocks on one axis.