There is no "right" length and the size of the dissertation is often based on the discipline. In the hard sciences (chem, bio, physics, engineering), there will most likely be a great deal of supporting data (from experiments) to report; same is true in some social sciences. However, the best advice I ever got was that writing less is harder than writing more- and it is infinitely preferred by dissertation committees. Write enough to show that you have an in-depth knowledge of your chosen topic, you've established your research question(s) thoroughly, outlined your methodology in a concise manner, and then spend as much space as you need to explain your findings. There will obviously be a considerable number of pages devoted to appendices (data and attachments) and a thorough bibliography- these don't really count towards your final product. (My first dissertation draft was 250 pages long- just text - after a great deal of work, I cut it to 85 pages and it was a much better manuscript for the culling).
It depends on the subject matter and whether the school or department has any specific requirements. I personally know PhD candidates who are writing dissertations up to thousands of pages, but that may be due to the fact that they are in literature majors and there are lots of quotes and references in their writings.