I do love sketchbooks and notebooks of all sorts whether they are handmade or commercially produced. I tend to gravitate to the medium size for both art journaling and travel journaling, somewhere around 5x9 inches.
My go-to notebook for this summer was the smallest yet. It was a beautiful, Rhodia 2"x3". The pages were coated and smooth, perfect for pen and colored pencil. I used it strictly for sketching wild flowers around Oregon in the early spring and summer. But I usually have 3-4 notebooks active at all times. Some are better for watercolor washes with heavier weight paper, and others are for everyday writing and tracking details about daily events, like the Moleskine below.