So, people get really hung up on the word counts, but the truth is they vary wildly. Scholastic's official figures for the U.S. editions are what everyone cites: 'Sorcerer's Stone' is about 76,944, 'Chamber of Secrets' is 85,141, 'Prisoner of Azkaban' clocks in at 107,253, 'Goblet of Fire' is a massive 190,637, 'Order of the Phoenix' is the longest at 257,045, 'Half-Blood Prince' is 168,923, and 'Deathly Hallows' is 198,227.
Thing is, these numbers aren't on the copyright page. They come from internal publishing data, and different editions (UK vs. US, paperback vs. hardcover, illustrated) will have slightly different counts because of typesetting. I once tried to estimate it by averaging page counts and multiplying, and I got a totally different number for 'Phoenix'. It's an inexact science unless you're running a text file through a counter.
What's more interesting is feeling that growth. The jump from 'Prisoner' to 'Goblet' is where it stops being a kids' series and becomes this epic. You can literally see the ambition in the word count.