One of the best ways I know to decide either way is to try making a plot map. Mind you, this is mildly hypocritical because on my current project (hopefully a novel if I manage to finish it), I started one, became frustrated (SUPER high puff levels), and decided to write for a while first. Two chapters in, I still have no plot outline.
But if you lay out what your plot is, how you plan to accomplish it, and sometimes set a plan for what happens in which chapter, it will give you a rough guess as to how long it may be. That can help you decide if it's enough to be a book or if you need to cut down a bit and make a short story. As a tip on short stories, though, they need to start fairly fast paced because it is a shorter amount of space for a lot of information.
Best of luck!