GSF Ghost told me on the server that one huge terrain (big enough for several TDM's) was probably around 2500 bricks. That's not much compared to the brick limit, it still gives you plenty of brick space to build.
He's also going to add two types of textures: one with normal textures (like the ones from maps), and one with brick stud textures.
The fact that he's made easy-to-make terrain is already pretty amazing, but he said he's going to be tweaking it and making smaller and larger pieces, aswell as different terrain parts such as more trees, rocks, plants, etc