Hi alfa11,
Hope you are feeling more settled about your structure and have some direction by now, but just wanted to say it can be a moving thing. I had a guiding structure I wrote at the beginning (think chapters with titles, then subtitles within the chapters) and then as my research progressed, I used it to help me work out when to stop reading and write, and also from time to time when I had to think about the overall direction of the thesis, it helped me to consider it as up for discussion. I restructured a few times through the PhD, reorganising the order of chapters or sections, taking things out and adding things in as I refined my thoughts on my topic.
I tried to think of my research design document as a working document, open for revision, if that makes sense? It helped to have a place to go back to to reconsider the big picture.
Hope that helps, and good luck!
BENKHALED - agree with allion2004 and TreeofLife - it depends on university and their regulations, most likely expressed as a word count rather than a page count, but follow their guidelines!