I get a lot of satisfaction from finishing a sketchbook. And it's immediately followed by the fresh promise of a brand new sketchbook to be started!
Last spread in my sketchbook. Next!
After decades of using various sketchbooks, I've fallen in love with the Stillman & Birn Zeta Series sketchbooks. I use the 8 1/2 x 11 hardbound ones almost exclusively. Sometimes they do feel too big (I'm perpetually on the hunt for the right sketching bag that will accommodate such a large sketchbook but not verge on being a suitcase) but I just don't like the 5 1/2 x 8 1/2 size as much. I wish they made the Zeta in 9 x 6 but they don't (yet?).
Other than recommending the Stillman & Birn brand, I hesitate to recommend sizes. Sketchbooks are such a personal choice. A sketcher develops a relationship with their sketchbook that is personal, that has to fit their way of working, their personal preferences, art materials, subjects.... and that elusive art bag...