The second smallest size binder clip, with maybe 6 or 7 cards in it. It’s maybe 8mm at its thickest and 6mm in most spots and is the footprint of a standard plasticized card.
I have two. One relatively small one that I take with me everybday:
Regular debit card
AmEx
Drivers License
Radio operator certificate
Airline bonus membership card
Some cash in in whatever is the local currency
The other one is larger that I usually keep in my backpack:
Passport
Two seaman's books
Vaccination booklet
Various visa-related paperwork
Some cash in frequently used currencies