Aside from the length of the work, there are many factors that separate a short story from a novel.
RIGHT FROM THE START
The first line of a short story should bring readers right into the action. The opening of a novel has more to accomplish. Even though the plot doesn’t have to be introduced in the first chapter, there are world-building tasks that normalize the setting, introduce characters, and establish tone.
There are fewer characters in a short story than in a novel. In general, short stories are limited to one point of view with under ten people running around. Since these characters populate a tight space, each one has a specific job to do and run the risk of becoming caricatures of their roles. Novels can dedicate whole scenes about character development without pushing the plot along.
Since length is the prime difference maker between short stories and novels, it is no surprise that the plot structure is affected. Plot, much like characters and setting, have to be laser focused in a short story. No detours. No extensive red herrings. At best, there is room for one or two good twists that are directly related to the advancing plot. This is a blessing as much as it is a hindrance because it is as easy to stray too far from the plot in a novel as it is to be overly simplistic in a short story.
Using these touchstones, you can better gauge whether your idea is best told as a short story or as a novel.