When you first come up with a story idea, how do you know if it's going to be a novel, novella or short story? If you only have a germ of an idea...just barely a spot, how do you know if you'll have enough material to sustain a 90K word project?
For me, the answer is pretty simple. After doing my character sketches, I do a chapter by chapter outline to see if I can pull together more threads of a story, and exactly where that story is taking me. This isn't the only way I work, but the most prominent. After looking over the outline I can usually estimate how long I can run the story for. If I don't have enough for a novel it will go onto the lists for novellas.
I have to admit that I don't always know when I start out if I'll have enough background or sub-plot to pull off a novel. I try to keep all that to a minimum with novellas, but to give enough depth that the story doesn't feel like only a surface telling. Also, I think it's something you have to decide before you start the project, since length- for me at least - can dictate the pace in which I let the plot flow.
How do you determine your story length? Before or after you start? Or when you get the concept?