https://www.jetpens.com/Delfonics-Carrying-Bag-M-A5-Dark-Denim/pd/37102
Delfonics Carrying Bag - M (A5) - Dark Denim<br /> $36.00
Perhaps a bit large, but could be used as a mini-portable office to hold a variety of things including 4 x 6" index cards for work on the go.
Depending on the scope of the notes that need to be taken, one uses either the A6format, or the next bigger one, which is A5 (21 x 14.8 cm), or the double sized A4 (29.6 x 21cm). After filling them with words, sheets of A5 size will be folded once, A4 size twice, sothat they return to our basic A6 size.
This is the first time I've seen in the literature the suggestion to write notes on larger sheets and then fold them up. This is largely only recommended here because of the standardization of the paper sizes in such a way that folding an A4 makes an A5 and folding an A5 gives an A6 and so on...
According to this, the arrangement consists of “wooden boxes with drawers that pullout in the front, and cards in octavo format ” (= DIN A5).
Luhmann's zettelkasten collection of cards was in octavo format, aka DIN A5 (148mm x 210mm or 5.8" x 8.3").