After trying a whole bunch of pocket notebooks, the Apica A7 has become my go-to.

It's 2.8 x 4 inches (10.2 x 7.1 cm), which is quite a bit smaller than your typical Field Notes-sized notebook. However, I've come to really like it better at this size. I'm rarely writing prose in these notebooks; I'm always writing short lists. So I usually don't take up a full line anyway. This tiny size leaves less wasted space on the page. It's also small enough to fit in a shirt pocket, whereas Field Notes often doesn't.

I've also been using the regular old Fisher space pen with it. I usually prefer something I can use one-handed, like a clicky or bolt-action, for my daily pocket pen, since it makes jotting things down that much quicker. But the size and simplicity of the space pen's design is just really satisfying sometimes.