Ideally, a novel is just one idea long.
Which is also the perfect length for movies, dinners, albums, advertising campaigns and blogs. Anything.

And the perfect chapter, scene, dish, song, advertisement or post, is one idea as well.

On a smaller scale; the perfect sentence adds one idea to the paragraph.
In its turn, the paragraph adds one idea to the chapter, which adds one idea to the book, which adds one idea to the trilogy.

Everything should contribute one idea.

If it does more; split.
If it does less; remove.