The overall thesis outline needs to be brief containing only each chapters title, and its primary headings. It is for you to organize how the thesis will flow and where each of the parts will fit. And since this is such a massive project, it allows you to break it up into more manageable bite-size portions. While there is no science to it, most dissertations have between 6-8 chapters, plus an introduction and conclusion, but this greatly varies. The guide-book mentioned above (Authoring a PhD) has a very nice detailed section on how to structure theses.
It also helps to write the estimated word count of each chapter alongside the title. That way, if you exceed the limit in earlier chapters you know that either you are too verbose or that the scope of your research is too wide and you need to curtail parts or entire chapters.
I have pasted below the very first version of my Thesis outline and the very last. Though the change may seem stark it is quite gradual if you see all the versions. Remember to number and date each version. I usually did this in the header of the document.