Back in those journaling days, I went through lots of notebooks with nice paper that could stand up to a fountain pen, etc. I wasn’t really a stationery geek, though I did love pens.
But for taking notes on the go, I had a Mead Five Star Fat Lil’ Planner. I replaced the silly planner stuff with a plain Fat Lil’ Notebook — spiral-bound and fairly thick, with perforated 3.5″ x 5.5″ sheets that could be torn off if you wanted to. A Cordura-or-something-like-it case that zipped closed securely, the spiral made a good place to tuck a pen or two, and there were a couple extra pockets and enough space to temporarily stash small items inside. Perfect for my needs. And they stopped making them almost 30 years ago, but you can still find a few on eBay.
I almost just bought one to recreate what I had then. But since pocket journals have become much more of a thing since then, what with Moleskine and Field Notes and so on, so I poked around a bit first. Field Notes happens to be 3.5 x 5.5, and there are several other brands in the same format with different types of paper, covers, and price ranges. A6 seems like a nicer size to me and there are several Japanese journal brands that use it, but after reviews and price comparisons, I went for the FN format with a relatively cheap, waxed canvas zippered cover and small Lochby notebooks which use Tomoe River paper that’s supposed to work well with fountain pens. Though I’m not sure yet whether I’ll want to use a favorite rollerball (Lamy Al-Star or Rotring Core) or a (probably Pilot or Lamy) fountain pen.