Minecraft wiki entry for World_border, associated text: Set
/worldborder set <sizeInBlocks> [timeInSeconds]
Sets the border to a square region with the specified size in blocks as the width and length. Optionally, a timeInSeconds may be specified such that the border grows or shrinks from the previous width to that being set over the specified time in seconds. If timeInSeconds is not specified, the world border changes immediately. To reset the world border size, set sizeInBlocks to 59,999,968. The border still grows or shrinks and the animation displays even if the game is paused.