Minecraft wiki entry for World_border, associated text: The reason the default world border is at X/Z ±29,999,984 is due to the fact that Dinnerbone put it 1 chunk (16 blocks) short of the edge to prevent falling through the world.[6]
Despite this, it became impossible to fall through the world at high distances in early 1.7 development, over half a year before the world border's implementation.
Were a Minecraft world wrapped around with the borders together, the resulting planet has a circumference of 60000 kilometres or 9549 kilometres in radius, twice as large as Earth.