1.You can just duplicate one of the existing terrain materials and then rename it and change the textures to your liking.
Note that your terrain diffuse texture needs to be highpassed.
2.I don't know about changing the player's size, but I wouldn't do that cause the lighting and the physics are set in real world scale so It'd give you undesired results.
Just set the scale properly in 3ds max.
3.You can export geometry as obj, but I don't now if that's optimal.
You can do your blockout in 3ds max, place it in your environment(scale it ,rotate it) and then if you want to change something or add any detail just add it in 3ds max and re-export your model.