My first novel started off as a ten page story. The first revision ballooned up to twenty pages. By my fourth revision, I was rocking fifty pages—that wonderful in between of not quite a novella, not quite a short story. I knew I had to make a choice: cut or expand. I chose to expand.
In retrospect, I probably should have cut. It would have been better that way. However, I do not regret my decision. I learned a lot from writing that first novel. It taught me perseverance, pacing. It taught me how to juggle multiple perspectives and plot threads. It also taught me that several plot lines I’d chosen to follow existed only to expand the page count. You’ve read books like this, I’m sure. You can tell because, half-way through, you’re thinking “Why am I reading about this?”
So how do you know if you should cut or grow? Look at the main tension of your novel—the main story thread. What’s on the line? What does the primary character want? How quickly can you resolve the tension. Strip your story down as far as it will go, keep only what is necessary. You may even want to do an outline. Some writers color coordinate their scene by scene outlines. You may not need to go to that extreme, but it may help.
Try the story as a 20 pager. From there, determine if you can reasonably keep it around that amount. Think of it as your first chapter. Can your first chapter stand alone as a story? If not, why not? What needs to be resolved?
If you want to expand to a novel, think escalator. How can you escalate the tension? don’t think the answer is adding new plot threads (though that’s one viable option). If you do, make sure that all threads weave together into something cohesive, maybe a doily or a blanket or a tablecloth—they should work together to accomplish a purpose as opposed to simply adding pages.
Then again, maybe you should color coordinate your outline, so you can visually see the tapestry you’re weaving.