Is there a consensus in philosophy and other humanities that PhD dissertations have to be at least 200 pages long? I've read a blog post written by Robert Wolff saying that for a philosophy dissertation to be considered good, at least in American universities, it should be between 200 to 250 pages long.

We could respond, of course, that it is not necessarily the number of pages or word count that makes a thesis up to par. If this is so, can you please cite a dissertation wherein a student was able to defend his central claim in less than a hundred pages?
The University of Minnesota library system maintaining electronic dissertations library contained 2,536 records for Phd students.

The range was incredibly variable (minimum of 21 pages, maximum of 2002), but most dissertations were around 100 to 200 pages.

The average number of pages per topic is : 


Source: https://beckmw.wordpress.com/2013/04/15/how-long-is-the-average-dissertation/

In fact, it mostly depends on the research area, the university regulation and supervisor research 'style'.