For nonfiction of all lengths I typically write single-spaced. Never really tried it for fiction except fairly recently for a short story. I guess it more closely approximates what a book would look like.

Page counts for fiction have been driving me nuts for some time. I used to use Times New Roman 12-point and knew that double-spaced, I basically aimed at a 400-page manuscript. But I switched over to Courier 12-point and now I'm not completely sure about page count, but I guess it's closer to 500 pages for 100,000 words. The word count is more what I'm aiming at, I suppose. It is in fiction (where I'm often paid per word anyway).