It frequently happens that I begin a novel with just a visual image of something, a vague sense of people in three dimensional space.
I think a playwright realizes after he finishes working on the script that this is only the beginning. What will happen when it moves into three dimensions?
It's my contention that each book creates its own structure and its own length. I've written three or four slim books. It may be that the next novel is a big one, but I don't know.