As the length increases, the possibilities for good novels initially increase. It may not be possible to write a superb novel significantly shorter than One Day in the Life of Ivan Denisovich. But eventually the possibilities for good novels start to decrease, because the length itself becomes an aesthetic liability. While one could easily have a series of novels that total ten million words, a single novel of ten million words just wouldn't be such a good novel. Indeed, it seems plausible that there is no possible novel of ten million words (in a language like human languages) that's better than War and Peace or One Day or The Lord of the Rings.
If this is right, then there are possible English-language novels with the property that they could not be improved on. For there are only finitely many possible English-language novels of length below ten million, and any novel above that length will be outranked qua novel by some novel of modest length, say War and Peace or One Day.[note 1]
So, there are possible unimprovable English-language novels. Are there possible unimprovable worlds? Or is it the case that we can always improve any possible world, say by adding one more happy angelic mathematician? In the case of novels, we were stipulating a particular kind of artistic production: a novel. Within that artistic production, past a certain point length becomes a defect. But is an analogue true with worlds?
One aspect of the question is this: Is it the case that past a certain point the number of entities, say, becomes a defect? Maybe. Let's think a bit why super-long novels aren't likely to be that great. They either contain lots of different kinds of material or they are repetitive. In the latter case, they're not that great artistically. But if they contain lots of different kinds of material, then they lose the artistic unity that's important to a novel.
Could the same thing be true of worlds? Just adding more and more happy angels past a certain point will make a world repetitive, and hence not better. (Maybe not worse either.) But adding whole new kinds of beings might damage the artistic unity of the world.