Shrinking World is an MCEdit schematic that you put in your world. Once its in place, you just play normal survival as the world size gets smaller.
Notes: This map requires snapshot 14w17a or later to work.
To place in your world, locate the center (x/z= 0) of the world and place the schematic in it. In MC, inside the schematic, set your spawn in the room containing a chest.
This is version 1.0 of the map, so many things aren't perfected. Expect an update to the map in the future.