It’s paralleled my move from bigger sketchbooks (letter size) to smaller (A5) to even smaller (A6) sketchbooks: Most of my notebooks are now pocket sized. I’ve had a pocket-sized Rite In The Rain notebook in my back pocket for a decade or so, as this all-weather book is great for quick notes. But up to a few years ago, if I used another notebook, it would often be at least A5 sized or more.
What has steered me towards small notebooks? Portability. I’ve got a bunch of different bags that I use in my day-to-day travels, and these bags vary in size. The best way to make sure I bring along my day-to-day notebook is to make it small, so it fits in bags like my North St. randonneur bag without issue. It’s the same overriding reason why my main sketchbook is now small, and why I have an iPad so I don’t have to cart around a laptop every time I want an electric device easier to use (and has a bigger screen) than my phone.
The biggest (ha!) issue with small notebooks is the smaller (ha! ha!) selection–you can find a great deal of variety in A5 (roughly half-letter) or A6 (roughly letter), but not so in A6, the “pocket” or quarter-letter size.1 Add to that my requirements that the paper be either grid lined (squared) or dot grid and be fountain pen friendly, and the options dwindle. Thankfully I found Rhodia, a French brand that makes some great paper and great notebooks. I can get A6 notebooks either hard or softcover, and they all have dot grid paper. Their Rhodiarama notebook is what I use as my daily driver for planning and quick notes.
I also like to use other pocket sized books for the daily carry. I usually have a small Rhodia notepad with easy tear-off pages for when I need to give a note to someone. I use a Hobonichi yearly pocket calendar too–while I plan out my week in my Rhodia, I use the calendar to note future appointments and such. And I carry around an A6 sized address book from Roterfaden. I have a bigger address book at home, but I fill up the Roterfaden with address of people I correspond with frequently, so I can write them postcards or letters while I’m on the road.
I don’t use a cover like I do with my sketchbook. Instead, I found a pouch-like bag made by Rickshaw, the Sinclair Model R. Rickshaw is well known for backpacks, shoulder bags, and bike bags (I’ve known a few cyclists who use them), but they also specialize in pen accessories. Inside the Model R I carry the notebooks I mentioned above, plus a collection of day-to-day pens. It’s a big snug fit, as I pack a lot in, but I make it work.
How about you? Do you have any notebook preferences? I’d love to hear about them!
1 Okay, technically the smaller A7 size is the true “pocket” size, but let me be me.
Urban Adventure League 
I don’t have super stringent notebook preferences; I’m just here to say that I appreciate you actually having tangible notebooks with real paper (and that you could rip out a page to give someone a note;) As a firm appreciator of non-digital things, I support this! heh.