If a finished draft is too long, or too short, the first step is to create a fresh outline, from scratch. This ensures the outline is accurate, and helps you to review the story as a whole. Ideally you should create multiple outlines at different scales. First, describe each scene in 1-5 sentences. Then, if the story is a novel, describe each chapter in 1-2 sentences. Last, identify the 2-5 key moments in the story.
All stories have three things in common: they begin with a status quo, use a conflict to disrupt the status quo, and end when the character either restores the previous status quo, or creates a new one. Depending on the scope of a story, the entire narrative may take place over a few minutes, or a character’s lifetime; generally larger spans of time lead to longer stories.