I can't imagine using one big file. To me that would just be overwhelming to deal with. I have literally a couple dozen files for my current WIP. The story has three parts, and each part is broken up into about 10 short chapters, so I have files for 1-1, 1-2, 1-3, 1-4, 1-5, etc., and then 2-1, 2-2, 2-3, and so on. Once I finished the first draft, I also complied all the part one chapters into a single "part one" file, and ditto parts two and three. I'd editing now and rechaptering some of it, so my original 1-1, 1-2, etc., files will probably go un-updated, but then I am saving my edits as "part-1-edited" so that I have multiple digital drafts to reference. (When I am done-done and ready to start sending it out for beta-readers or publishers, I will put everything in one big file.) Also while I'm editing if I'm deleting a large chunk of text or even just a sentence I'm particularly fond of, I'll C&P it into a file titled, for example, "2-4-deleted" in case I want to go back and put it back in again later. I almost never do this but for the few times I have it's been nice to have the "deleted" files. With the current rechaptering business it's getting a little messy with all the chapter files and deleted files, but this still feels like the easiest and most logical for the way my own personal brain works, although typing all this out makes me sound slightly crazy...