Keep It Simple
Although it is wonderful to imagine the most incredible and complex stories, there will always be limits. Even the biggest studio blockbuster has a fixed budget. For a personal film, the limit is the amount of time that you alone can give to the project. Most projects fall somewhere in the middle.
With any project, there is always the tendency to bite off more than you can chew. Creating a story is easy. Actually producing it is another matter altogether. It is always best to keep the time and budget constraints in mind. Knowing how much time and effort are required to produce a particular film is knowledge gained mostly from experience. The more films you make, the more aware you will be of the time and expense involved.
For students who are creating their first film, the best advice is to keep it small. This means sticking to a handful of characters and a situation that is manageable. Usually two to three minutes is plenty, and four to five minutes is ambitious. Films over five minutes might require some outside help. Remember, the classic Warner Brothers cartoons were all only six minutes long.
In any project, simplicity is typically the best way to go. Most of the best short films have very simple plots and stories. The Pixar shorts are a great example, in that they all have only a handful of characters and one simple conflict. Another case for keeping the story simple and the number of characters small is that you can spend more screen time developing each character. Isn't "character" what character animation is all about?