How do authors use the length of a story to re-enforce or enhance the theme or action?How do authors use the length of a story to re-enforce or enhance the theme or action?
This depends entirely on the author and the period. The short story is a relatively new genre that originates in the oral tradition (story telling) and was originally designed to be published in magazines. Early on in the twentieth century the short story followed conventions such as having an exposition, climax, and denouement. Additionally, many had didactic value, which means they attempted to teach the reader a lesson or make a point. Later on, with the rise of Modernism, conventions faded into the background, or were disregarded entirely. A good example in American literature is John Updike. His stories are sometimes really brief and uneventful, but yet often make a powerful point in the end.
The key characteristic of the short story is ,however, that they are more straight forward than a novel. Usually, they deal with a single incident, have a limited number of characters, and take place in shorter period of time. Short stories that are published today are quite often written to fit specific directions for magazines or fiction contests and thus have to fit a certain word count. Only published authors have the liberty to write stories of their own design that disregard requirements of length.
So when you attempt to speculate on the relationship between length and content, you must ask yourself who wrote the work and when.
One can use even a very short piece to express a concept which is deep, or may evoke long thoughtfulness on the reader's part. But if there is sustained activity over a long period of time in the story, or several characters with interacting relationships, the author must write a longer piece. Action which takes months or years can be drawn into longer stories or novels, giving the writer time and space to develop subplots, interacting relationships on levels below the obvious activities of the characters. Use of literary devices such as foreshadowing can be used.
Basically, very short stories make one profound point quickly, a short story may develop an action or character in more detail, a novella or novel may be used for deeper and more revealing study of human behavior.