A dissertation can be viewed from many different angles.
My experience is that most people are thinking only of the argument.
Most writers are focused on the need to get the argument and the ideas right.
Obviously I believe that this is important (see yesterday's post), but the argument is only one way to look at the dissertation.
In order to generate a clear vision of what you're trying to create, it can be helpful to think about more than only the argument itself. What sort of rhetorical and argumentative structure are you going to use? How long do you want your work to be? To whom are you addressing the work? To whom is the outcome important? These are all questions that generate different perspectives on the work, and in combination they can help you focus your efforts as a writer.
Thinking about argumentative structure and rhetorical structure is probably the aspect that we most often think of when we're not thinking about the argument itself. This is the outline that we work on; it's also related to other stylistic choices.
Thinking about the length can be frightening. If you think "I have to write a 50 page chapter" that might well be intimidating. If you think to yourself "I have to write a dissertation and it's going to be 200 pages or more," that's definitely intimidating. the numbers break down quickly as you start to outline and develop your plan for your structure. If you have to write a 50-page chapter and you recognize that the chapter has three main parts, then each main part will be 20 pages or less. And a 20-page isn't nearly as intimidating. If you think that each of those 20-page sections can also be broken down into pieces, too, then you start to have lots of pieces that are each relatively small and contained. And, of course, if you have a good overall vision of your project, and you keep in mind how those little pieces relate to the larger structure, then the individual pieces don't get written as independent little essays, but rather include the ideas that tie them into the larger fabric of the dissertation.
Thinking about the audience and who might read the work also helps make choices. It helps with making choices about tone and style. It also helps make choices with material--knowing your audience, or imagining an audience, can help in thinking about what information would interest that audience, it also helps think about how much background information you need to give to the audience.
Thinking about who is affected by the results of your study is also important--this is similar to, but not identical with, thinking about the audience. It may be that the audience serves the interests of those who are affected and impacted. It may be that those who are affected or impacted are not directly related to the audience at all, but that doesn't mean that the information revealed will not eventually come into the hands of those who could use it in favor of those impacted.
By taking different perspectives on the work, and by looking at it from different angles, a better-defined idea of your goal can arise, thus aiding you, the writer, in trying to plan your writing and focus your energies as you write.