Someone asked recently in a writer's group:
"When you initially start working on a story do you consciously make a decision as to whether that story, when finished, is going to be a short, a novella, a novel, or even series of novels? Or do you just go with the flow and let the telling and the actual execution of the writing of the story determine its length?"
Well for myself I have almost always started as if writing flash fiction. But I have never finished it as flash fiction. Even when I really was writing flash fiction. Whatever length I start shooting for, I nearly always have exceeded that. I've gotten better at it though and what I notice now is that I'm compressing the action more, so that there is more happening in less space; while trying to make the prose be more, with less. I've had to turn out a piece that was a specific length for a publication or something, but that's usually non-fiction. Either way, I can do it. It's tougth sometimes though.
I get to the end of short stories feeling like I'm running a marathon when I'm actually running a sprint. Or perhaps vice versa as in a marathon one should pace oneself, yet, I feel more like I'm running full out to the finish in an attempt to hurry up and complete my story.
Otherwise, I'm simply writing a novel, which I should have planned out more. Sometimes I start by knowing the ending and I write toward that. Sometimes I have a concept and start with that and write to the end; but then the middle always shifts as I add to the beginning and the ending as I think of clever, before unseen directions to go, which can be exhilarating as it feels like someone else is throwing in ideas that I find exciting.
It's also kind of like I'm moving through the story along with the reader. Except, that I'm there first. It's a bit sometimes, like riding on a small ancient, seafaring boat and I find that I'm the only one there. I try to stand in the middle of the deck on the boat, with no navigator and, while in a storm, I'm doing all I can to stay in the middle so as not to get swept overboard.
Or, it's simply like being delusional and I'm just writing it down.
So, typically I just start writing and see how it goes. If it starts to feel like there is a lot there, I keep going; if not, it becomes a short story. Lately I've gotten more into novel length works so I know what I'm getting into when I start. What I've detailed here has been how things have been for most of my life. But now things have changed. How it will pan out in the future, well, we'll all see that soon enough....