As I understand it there are 48 tiles per region tile, and 16 region tiles per world map tile so...
48 x 16 = 768
768 x 257 = 197376
That puts a large world at 197376 x 197376 which is 38,957,285,376 tiles in a world. Given tiles have no specific dimensions though, its hard to gauge just how big that really is.
Feel free to correct me if I'm off with my calculations or if this wasn't what you were really asking for
Using the assumptions that tiles are 10 feet (3 meters) tall, because that is standard story height, and that all tiles are cubes, I've been running on the assumption that the current tiles are 3m cubes (27 m
3).
If we have 197376 tiles across, then it is 592.128 kilometers in either direction, or 350,615.6 km
2 surface area (which does not account for any curvature of a real planet, although DF planets are apparently cubes.)
The earth's surface is roughly 510,072,000 km
2, for comparison.
Or in other words, we're several orders of magnitude off from a full earth-planet size, but maps are only supposed to be regions that just happen to have the only life on a planet, presumably with some kind of force field that prevents all migration across the edges of the region, so that when something goes extinct within a region, it is extinct for the whole world.