Sphere, a minecraft adventure map, is claiming to be the largest and most polished minecraft adventure map of all time. With an isometric render of the world that will take your breath away, it might just be true. Certainly the Sphere itself is larger than your average adventure map, so imagine how big the world itself must be.
The creator truly has outdone him or herself, as mentioned in this post about the possibility of this map having been created by Marvin the Paranoid Android. At any rate, it is certainly worth a download, if only to see what something large looks like.