Well, ideally, a story is as long as it needs to be, and no longer.
There are norms that have developed over time, and which are more or less inculcated into us due to our exposure to stories in their typical media. For example, a typical feature length film of roughly two hours has between forty and sixty scenes. Formatted according to industry standards, a screenplay has approximately as many pages as the finished movie would have minutes. In terms of plot events, some people in Hollywood believe that a commercial movie should have exactly forty (which in Beemgee’s plot outlining tool would mean exactly 40 event cards).
Content and form may be mutually determined, to some degree at least. A short story is usually considered such if it has less than 10.000 words. By dint of its length, a short story probably concentrates on one character’s dealing with one specific issue or occurrence, and is unlikely to have subplots or multiplots (that is, be about more than one protagonist).
A piece of written prose fiction between 10.000 and 50.000 words is often considered a ‘novella’. This is a sort of hybrid between the short story and the novel. The narrative of a novella is likely to cover more ground – that is, relate a longer and more complex set of events – than a short story simply because it is longer than a short story. But to state that a novella perforce has more depth or more action than a short story would be a meaningless generalization. What is likely is that the focus in a longer narrative such as a novella is on a string of occurrences rather than the story revolving around the meaning and effects of a single occurrence.
By this set of definitions, a novel is any prose fiction longer than 50.000 words. That kind of length allows development of a complex interaction of occurrences and ideas, for instance by including subplots showing the stories of several characters.
Outlining a work before writing it will give the author an idea of what form is suitable for the material. But an outline needs to be kept in sync with the development of the story that takes place during the writing process. Henry James planned The Ambassadors to have about 100.000 words in ten parts divided into two chapters each – he ended up with over 160.000 words in twelve parts with a total of 36 chapters. That was, in the end, how long The Ambassadors needed to be in order to get the entire story across.
Develop your story:
Meaning and subtext arise from story structure as much as from the way the story is told. The emotional reaction of the reader or audience to the characters and their plight – i.e. to the story – is ultimately the most effective carrier for meaning. So perhaps the most interesting or important rule of thumb is this: Any scenes, events, characters, chapters or even words that are extraneous to the story – i.e. that do not help it move along by conveying more plot or relevant details about characters or their environment – are probably better removed.