........

You have a couple options:
http://en.wikipedia.org/wiki/Megatexture

But more likely, http://en.wikipedia.org/wiki/Texture_Splatting
But you can't texture really with a single texture, you're going to need at least a couple.
Your other option is to assign vertex colors that correspond to certain textures, maybe?

I suppose if it was a technical requirement for some reason, you could arrange various textures onto one map and uv the terrain accordingly- or- texture in max using multiple maps, and bake to a second unique uv set on one large map? much easier to use multiple textures as mentioned though