I'd look at a few finished ones from your department/school and see what the local convention is.

Our University dissertation handbook specifically says 'don't forget your advisor' in acknowledgements, and it is expected in my department that you thank everyone on the commitee. Acknowledgements are also considered a forum for relatively free expression: mine are over five pages (and includes all kinds of folks from family to LJ friends list), with three paragraphs detailing helpfulness of various committee folks.