I have a large collection of sketchbooks and journals beginning in 1979. Back then I used an A4 Architect's and Designer's Diary. Now I use a Lett's weekly planner for my scheduling and journal entries, a Aquabee spiral sketchbook for drawing and a pocket Moleskine sketchbook for both. This photo also shows my Canson Montval watercolor sketchbooks and my collection of Canson All-Media sketchbooks.
I recently discovered the very large Moleskine sketchbook, shown at the top. It is 12" x 16.5" and is perfect for capturing large landscapes and nice horizontal vistas. You can see its size as compared to my pocket version. I usually keep this one in my car and just pull it out when there is something big to sketch.