Over the years I've seen discussions in magazines and on blogs about the intrinsic difference between a short story and a novel. This might seem trite but the answer is more than just length. A short story is usually one or two scenes. That's it. And don't waste your time telling us he had green eyes and brown hair and a cute mole on the tip of her earlobe. The clock is ticking. You need to begin with a conflict. The protagonist has to overcome a challenge and we'd better discover that pretty soon. Normally short stories are around 2500-3500 words, so don't dilly-dally. I'm not saying you can't set the scene with a full moon or a distant howl from a coyote. That's okay, as long as it's taking us somewhere.
I've been fortunate enough to have published quite a few short stories (many are available in full on my website) and everyone of them starts with dilemma. A clairvoyant patient visits a psychiatrist because she hears aliens discussing the destruction of the planet. A man gets arrested because he can't handle the stress of his job and that's the only place he can find respite. These are just two of the ideas I turned into stories.
The novel, however, requires an entirely different pace. Now just because you have more time to develop your characters please resist the temptation to describe your protagonist through a glance at a mirror, or someone commenting on their green eyes, brown hair, or cute mole on the tip of her earlobe. You'd better start with some form of conflict pretty quick and keep that tension going throughout the novel. The difference is the conflict probably won't get resolved for a few hundred pages instead of fifteen. Does anyone know what Jack Reacher really looks like? We know he's big, and that's about it. Lee Child doesn't waste time with such insignificant details. We know the shape of the road he's travelling because it will become useful at some point forward.
Now all of this pertains to most genre fiction, so don't get all tangled up trying to make sense when it comes to romance novels, because that is a completely different animal all together. Let's face it, the details of someone's anatomy are usually the most important part of the story.
Of course the best advice is to study the short story before churning one out. There's plenty of great fiction in today's literary magazines and journal's. Just remember, start strong and finish strong. What did you think I was going to say on a blog named Strong Scenes.