I can also add that third dimension for some extra on-screen or real life punch.
Here are some forced perspective miniatures I've done for a few independent films:
Many people assume I'd built a full sized horse for this shot, but it's only about 12" across.
The Soviet missile silo looked awesome on film, especially with the actors interacting with it.
Yet the missile itself was just a vodka bottle and the actors were about a quarter mile away from it - so far we had to direct them via cell phone. This shot was done entirely in-camera.
and this is a small set piece carved from foam:
This is an alien skull hand carved from foam. It's probably about 20" long.
And here we go the other direction with full sized sculpture:
This statue was 7' tall.
Soon I'll be building a miniature steam cargo ship for a series of three short films, but I suspect 'miniature' will just be a technical term as I suspect it'll end up being pretty big. Watch for a blog entry with photos once those projects have all been screened around late April 2010.