What can YOU say in six sentences?
I say go where the story takes you. If you decide to start with a short story, even a 6S, and it develops into a novel, keep going. Working with 6S has taught me a great deal because I use to start saying its going to be a novel and it goes nowhere. Here I know 6S and I complete the task and either it remains 6S, nothing more need be said, or I decide I can take this somewhere further and...
To each his own.
I definitely agree there is a different feeling when you finish a short over a novel. I wouldn't say I'm disappointed more often with one though (only if the ending is bad).
I think it depends on the person. In my opinion, crafting a fantastic short story is every bit as difficult as a full length novel. You have to develop your characters and create and resolve a conflict with much less room. I think the important thing is knowing what kind of plots/stories would do better in which form.
When it comes to writing, Stephen King strikes me as a guy who knows what he's talking about.
Yeah, I think I'm starting to agree with him.
What I'm getting out of the comments and King is that you have to listen to a story and your characters instead of forcing it into whatever form (or even genre) you want it to be. I think I've been guilty of that before. Glad I'm realizing it though...
I like to read short stories, and I think they hold a special challenge for writers. The same is true for all forms, I guess. I like to write sixes, but I'm thinking of branching out. Just thinking.
Once upon a time, a really great editor (mine, thank the writing gods) asked the rhetorical question: How long is an essay? And of course the answer is, "As long as it needs to be." This applies across the board. How big should your canvas be for that painting?
What I keep doing is WRITING, and staying firmly in the process. The product reflects that. My first novel is an epistolary account of an old man's life written in his final days. Turned out to be a novella rather than a novel. But, it started with two 6S pieces I HAD TO WRITE. And yet, neither of them made the cut when the novella was complete. All of that to say... just write. The product will reflect your process. Don't be formulaic. Be true to the voice, the writing, and the story.
And.... done beautifully and well, short stories are powerful... and very difficult to write. A whole different animal from a novel.