There is a map that dorpond did that I think was a couple miles square so yea, no size limits, or if there are they aren't really significant.

There are several tile sets that are currently in use that you can see at the RPTools forums. Users are making pretty large maps inside MT natively.

Oh, and there is a great map of the Temple of Elemental Evil floating around somewhere if you want to see MT handle a really large map.