In most cases, researchers must confront the issue of subject selection, although in some where a title is given the emphasis will be on which aspects within that area should be pursued.
Selection of a dissertation research project will usually be confined within the boundaries of particular disciplines even then the possibilities are vast.
It is usally a good idea to get into the mindset that your research will be fluid and potentially change over the time you spend completing it. Therefore your initial subject selection might also change over time.
As your research progresses you may find yourself constantly focusing and refining your ideas. How much this happens will depend on the nature of the discipline and the appropriate research methods.
You might find it handy to narrow your topic area down by constructing a set of lists:
List 1: Topics of interest
List 2: Personal strengths and weaknesses
List 3: Topics of current interest in practice
List 4: Access to data
List 5: Research limitations.
Ultimately you have to make the call on something that will keep you interested for the duration of the project, will hopefully play to your strengths and have suitable access to current materials!