Here's the script and shader I use for applying normal/spec/gloss to the terrain. Readme's in there with instructions. http://www.farfarer.com/temp/unityTerrainNormalSpec.zip
Any limitations / specs for the diorama? The only limits mentioned are for the char and the vehicle. ANSWER: Same as everything else: 4096x4096, no triangle limit.
It's unfortunate that so many of us would love for such a game to be made, but apparently not enough for it to be gangbusters profitable. see - Spec Ops: The Line