And you thought the Game Boy Micro was small. Jeroen Domburg's custom lil' handheld here is small enough to fit on a keychain. It's so small you can't even read the text.
Domburg spoke about his creation at the 2016 Hackaday SuperConference, explaining that the tiny Game Boy was made using a 3D printer, ESP32 micocontroller and a tiny colour LED screen.
If you get to 39 minutes, you can see that, yes, it also plays Doom. Barely.
Since the device is too small to use actual cartridges, it downloads games from a network then boots them via an onboard Game Boy emulator.
The whole thing looks totally impractical and painful to play and that's why I love it. This isn't an exercise in ergonomics or market feasibility, it's a guy saying, "I want a Game Boy on my keychain," then just going out and making it happen.