Here's a list of the books in order of length:
1. Harry Potter and the Order of the Phoenix: 766 pages
2. Harry Potter and the Goblet of Fire: 734 pages
3. Harry Potter and the Half-Blood Prince: 607 pages
4. Harry Potter and the Deathly Hallows: 607 pages
5. Harry Potter and the Chamber of Secrets: 251 pages
6. Harry Potter and the Sorcerer's Stone: 309 pages
7. Harry Potter and the Prisoner of Azkaban: 435 pages