How to convert Object to terrain when object is big and gets split into multiple objects?

I put a different material on each split mesh in the image above. As you can see one large mesh was split into 15 different mesh's. What I want to do is make a bunch of 256x256 terrains tiled from this one large mesh. But with the way it is now the object2terrain script does not work. Is there a way to tell unity how you want the terrain divided up? Can I select 256x256? Or is there a way to make 256x256 terrain tiles from this?
Thanks.

 

Nevermind. I rewrote the object2terrain, and just did it in chunks.