Here is a simple way to help determine if your idea is more appropriate for a novel-length work or a short story.
- A short story shows a character in a state of flux. While other characters are present in the short story, it generally deals with one character's situation. Hence, short stories are almost always character driven.
- A novel has a broader scope. Not only will it deal with a character's trajectory, there will be substrata and superstrata as well. You'll often find a number of leitmotifs, recurring ideas, or details that appear again and again. A novel's larger idea (or theme) is many-tentacled and won't always stay between the lines.