The story arena. This is the arena, uh, in which your story plays. Yeah, simple and straightforward, right? Every story - fiction or nonfiction - has this. We need the where and the when. A friend talks about her Mother's Day and begins with, "So at my house this afternoon..."
It's true that some stories have more elaborate arenas than others. Hell, the Star Wars films have a whole goddamn galaxy upon which Skywalker Ranch can ejaculate loads of computer-generated wonders. Others, such as My Dinner with Andre, need only a table at a restaurant.
Exactly how big of a story arena you're going to need is up to you and the story that you plan on writing, and far be it for me to go telling you how you should even put it together. But having just finished getting the story world pinned down for Undead and Inhuman, one thing that I can suggest is that you don't go overboard.
During the making of The Phantom Menace, George Lucas said that everything had to be designed and thought through right down to the forks. I can understand this because this was done while the script was being written and not everything was pinned down and set in story. Add that to the visual medium of film, where you do need to know what everything looks like, and it makes detail much more important.
You don't have to worry as much about that with fiction and prose. I was at Pasadena's LitFest yesterday, and one of the panels had Amber Benson of Buffy the Vampire Slayer Fame. She touched upon Anne Rice's work and Rice's penchant for incredible detail, saying, "That's interesting spending a page and a half describing a rug that never appears again in the book." That's a page and a half that could have moved the plot forward or developed character. Of course, there are stories really do need this kind of detail like The Picture of Dorian Gray because it has a lot to do with the experience of the senses. In such cases, description and detail are part of the story itself rather than just the icing on the cake.
My advice for tackling the story world is to keep it as simple as possible. The worldbuilding notes I got for Undead and Inhuman are roughly twelve pages, and many of the different facets of that universe are condensed down to about half a page.
There are two benefits to this. First, it means you don't have to go crazy and pull your hair out worrying about - how, for the love of Cthulu! - would vampire homes be built in the future! I got that tiny example down to four brief sentences. Second, and I've said this before, your story bible shouldn't be fully formed before you start writing. You should have just enough information down to see that there's a house on the other side of the street shrouded in fog. The detail of the porch, the color of the front door, the number of windows, those are things you'll find out as you get through the writing.
And as you do, you include them in your notes and keep building them up.