The R is my friend blog used some nifty coding to analyze 2,500+ dissertations (though only those since 2007). (via)

Most were between 100 and 200 pages.

The below boxplot shows the length by topic. It could have used a legend to verify, but traditionally, the vertical line is the median, the colored areas are the 1st and third quartiles, the horizontal line is the range of extreme values (1.5x the inter quartile from the upper and lower quartile, and the dots are outliers (greater than 1.5 the inter-quartile range). Make sense? Of course, page length doesn’t actually indicate much of anything… so this whole exercise is pointless – just like most PhDs. 🙂