While perusing writers' markets, one offered a tip to writers attempting to break into that market explaining that the most frequent flaws are stories that don't have a proper ending.
Many times writers focus on perfecting the hook or lead in their story but think nothing of rushing the end. Without an effective hook the reader loses interest, but no matter how well you hook them if the conclusion doesn't deliver, the manuscript will most likely land in the rejection pile.
An effective end wraps up each emotional and logical thread spun throughout significant events and character activities in the plot. The conclusion needs to satisfy the reader. By satisfying, I mean it resolves every shred of conflict and tension without being predictable. Instead of leaving the reader confused, a well-written end allows the reader to ponder how cleverly the author knit significant plot threads together, leaving them amazed they didn't see it until the end.
The End Must Be Visual
Writers work to sharpen skills to create their entire story using active language that brings the story to life with the use of strong verbs. The end is no different. The writer's goal should be to create a lasting impression that provokes further contemplation. Use active language that leaves a vivid impression. Don't tell the end; show it. A visual impact lasts because it burns an image into the reader's memory.
Where The End Starts
The end starts with a calamity or disaster. The Writer's Journey by Christopher Vogler breaks the writing process into three acts. I highly recommend this book as a guide to include all the elements in the Hero's Journey Model. Steps outlined in Vogler's book help bring the story to a clear resolution. The crisis leading to the conclusion starts within the second half of the second act, builds to a climax in the third act and leads to the end.
Knowing when to end the story becomes a matter of instinct. As a writer, you've given birth to characters and settings. You know every detail encountered in the process. Drawing them to a conclusion will provide a feeling of completion. It will feel right.
The length of the story will be dictated by the story line. At times, what starts out as a short story may develop into a novella or even a novel once characters take on lives of their own. The trick is to write until the story is finished. Editing and rewriting can bring it into line with word-count guidelines after completion of the first draft.