My fiction writing has several stages. First, I'll usually come up with an interesting setting: Canoe trip, Wordcon, middle school with ghost girls, etc. I then toss a generic character in, give them some sort of personality, and see what they do. Toss in other characters, let them interact... then realize it's a silly idea and start thinking of something else.
I currently have an idea that has passed this stage, having made it all the way to a full mental first draft: I've let the characters live the equivalent of a month or three, seen what they've done, and found it worth writing about. Even came to an ending, although to be honest several endings have suggested themselves depending on how much I want to piss readers off. I'll be out of town next week, but after that will attempt to get this out of my head in useable form. It may change before then, naturally, as this was the first run-through. I have no idea about length, but it won't be epic (nor the start of an epic, although apart from The Waifs that's always the case). I may finish it and post it in one chunk. We'll see.