[The Green Ringwraith]

What I think they did for most scenes is that they film one actor, say, Gandalf, with the scene. then they take Bilbo and film his part with a "green screen" background. Green screns are used like this: the green, when taken in to a computer editing porgram, can be reased as a tranparency. So, the only thing left is the actor. Place the two scenes over each other and voila! You have the sizes. Maybe they filmed Bilbo's scene and downsized it, then put it over Gandalf's. (This is at Bag End) and used the same technique for others. I will know a bit more after a few months- I'm taking broadcasting in school!