Rasberry Pis and similar are sometimes sold as credit-card sized, but they are not portable, really, have huge Ethernet ports and no display or keyboard. Those are on the too-big side for what i am looking for.
In the other hand, you could say that SIM cards or USB devices are technically computers, but they lack a user interface. They are too small.
Is there something small with a usuable display and input method; something that looks like a calculator or tiny smartphone? Does not need a battery (although that’s a plus). Does not need WiFi or 5G (although that’s plus). Needs some standard method to communicate with the outside world (USB, Bluetooth, NFC, Wifi, …). Should easily fit in a pocket, wallet, purse.
Ideally something that can run a minimal Linux or Android?
Consider the cardputer Link. It’s intended for hobbyists who are willing and able to write programs for it but the hardware is roughly what you described.
Sexy and very tempting at the price tag. For protyping that’s ideal and it demos what’s possible.