Because short stories are, well, short, they generally do not have enough space to cover all the points necessary to make a grand argument story. Therefore, there are two techniques that are most commonly used.
The first is to cover the same "breadth" as a grand argument story, but limit the depth of exploration. This type of short story has the "feel" of a larger story (e.g. The Turn of the Screw by Henry James and A Good Man Is Hard To Find by Flannery O'Connor), yet still falls into the short story category. This type of story will have all four throughlines represented in the story (Objective story, Relationship story, Main Character, and Influence Character). Frequently one or two of the throughlines are more heavily emphasized, but all are present. The economy comes in limiting the depth of exploration.
The second type of short story is to go to the full "depth" as a grand argument story, but limit the breadth of exploration. This type of short story seems to focus solely on one throughline (generally the Objective Story or the Main Character) and may only hint at one of the other throughlines (e.g. The Lottery by Shirley Jackson and The Pit and the Pendulum by Edgar Allen Poe). This type of story is frequently used in the shorter stories and often has "trick" endings (think O. Henry or The Twilight Zone). Longer form works don't support this type of story nearly as well as the short form does.
Now, how can Dramatica help you develop your short story idea?
The first thing you should do is determine a storyform that conforms to the part of the story that you know. Even though you may be limiting the breadth and/or depth of the exploration of the storyform, it is important that what you do explore fits together well. Starting from a complete storyform will dramatically reduce potential logistical and emotional "holes" in your finished work. You can achieve this fastest by using the Story Engine or the Quick Trip path in the Query System. However, most of the query system paths can be used to create a single storyform. Experiment a bit with the program to find one that asks the questions you are most interested in. In fact, only answer the questions you want and, if you desire, answer some questions in one DQS path and switch to others to answer other storyforming questions. Dramatica doesn't care how or where you make the choices, it just needs for you to make them.
Next, pick which type of short story you want to do and illustrate ONLY those parts that are relevant to your finished work. (You CAN illustrate the entire storyform, but you may end up not using a lot of the material because it won't "fit" into the limited space.)
Lastly, weave together the pieces of the story that is to appear in the story. This should be done in a word processing program. You should end up with a story which, even though not fully drawn, implies a larger picture than the sketch explored in the finished work.