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?