What size are your cards?
I know Luhmann used thin A6 sheets of paper.
I'm considering using A7 index cards for my main box, and A8 index cards for my bibliographical box (yes, I want to build an analogic Zettelkasten). I presume that single cards will be sufficient for my notes in most cases, and if I need some more space I can always make a "double" note by using two cards (or even more).
What do you think? What size are your cards?