The closest option is probably the GPD MicroPC. It's 6", but pretty damn close to the ideal of a truly tiny PC. I think they're making a successor to this finally, dunno what the status is.

https://www.gpd.hk/gpdmicropc

Thanks for the link. It looks pretty cool. Form fact wise it looks to be competitive with an Android phone and a portable keyboard. So it's not clear immediately which is better.

I use termux daily, and you can stand up pretty much anything including a production grade PostgresDB. It's not a toy.

As an ex Psion 5 user, I've often been tempted by the devices from Planet Computers: https://store.planetcom.co.uk/products/gemini-pda-1

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As an owner of Cosmo, I can confirm all of the above. The form factor is great and the keyboard is amazing, but the software story kills it all dead in the water. I wish someone could just do a dedicated Linux machine like that.