How to Make the Border come in to a certain coordinate
Solution 1:
What you are probably talking about is the World Border.
To make the border centered around a block, use the command
/worldborder center <x> <z>
Then, to set the size of the border that you want you use the command
/worldborder set <sizeInBlocks> [timeInSeconds]
(Notice the [timeInSeconds] attribute. If set to a number it allows you to have the world border actually change its size slowly instead of instantly)
See World Border on the wiki for more information. It has commands explanation as well.