I really like Field Notes sized notebooks. They are small enough to be portable and carried in your pocket but large enough to actually use. I find the passport sized books too small but these are perfect.
This one below reminds me of a Hobonichi cover, and comes from Cult Pens
Made for Field Notes by Iron and Resin of California, the Field Notes Everyday Carry is a handmade notebook cover made from high-quality top grain cowhide leather in dark brown.
With a slightly waxed smooth grain texture, the Everyday Carry Memo Book Cover is a wonderfully tactile accessory that’ll hold a notebook, a pen, credit cards, cash and any other day-to-day ephemera you may need about your person.
The dimensions allow the Everyday Carry to slot easily into your back pocket to make it a true everyday carry item.
Supplied with a set of three mixed Field Notes Classic Notebooks as well as a Field Notes branded ballpoint pen with black ink.
Here is another one, also from Cult pens which I really like!
The Pony Express notebook pouch is a rugged storage solution for Field Notes, designed after an antique U.S. Postal Service leather postage satchel.
Handmade from premium leather, the Pony Express pouch will age gracefully, getting darker and more supple with age.
Dimensions: 149mm x 107mm x 20mm. Fits three Field Notes notebooks.