So what makes it a story instead of a scene?
The critical litmus test to determine if you have a complete short story instead of a scene, is character change. Is the character a different person at the end of a story than he was at the beginning?
In a scene, the character might struggle with an assault, might fight off the bad guy and triumph, but if he is the same person at the end that he was at the beginning, then this is technically a scene and not a story even though we have a clear conflict and resolution. Remember, scenes need conflict and resolution too. So it’s fine to begin with your climax and imply the back-story, but let your character change in some way. Example: He or she views the world with a slightly different perspective. That is what ‘change’ means.
Many new 'short short' writers run into trouble in two ways.
1. They try to follow the classic short story form: building toward the climax and then resolving it. But the shorter your story, the less time you have to build. When you are writing a 500 word story, you have no room to build toward anything except the resolution.
2. Or, they begin with the conflict; for example, our dragon fight, but the character doesn’t change. But if our dragon fighter realizes that dragons aren’t monsters but are actually intelligent beings whose beliefs and wants match his own, and his people have it all wrong-well, there you have a story. Our character is no longer the same person as he was in line one. His mind has changed.
Often when new writers start with novels they get discouraged. It's too big a project. Start with something smaller and build up. Think about writing a series of shorts with the same MC and and main plot goal, with the intention of merging into a novel eventually. Each short could resolve its sub-plot at the end, but its main plot isn't resolved until the last story. Wouldn't that be something more sustainable and fun?
I've had readers tell me they were able to complete NanoWrimo 2010 using this technique, and it relieved much of the stress. That's the name of the game, isn't it?