Is there a consensus in philosophy and other humanities that PhD dissertations have to be at least 200 pages long?
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'.