Well, now I had to make some research.
From my experience a FHD (1920x1080) screen in landscape is good for writing a A4 document in portrait mode if the text isn't too small.
So lets assume 1920px for 210mm., which is 9.14px/mm or 232ppi (pixels per inch).
A A4 page is 14.4" (for reference)
Now lets assume you want to view your page in portrait mode, then the screen would need 2720 by 1920 pixels.
possible panels:
2760×2070 11.5" (too small?)
3200×1800 25.3" (too large to be portable)
2200×1650 13.3" (only 206 ppi)
That would still leave the need to build a housing and build a text-writer that is optimized for this application to make writing at least somewhat comfortable.
There appear to be a handful of devices using the last panel, but it is discontinued and most are sold out. And even if you manage to get one, you would still need custom firmware.
There does appear to be a new one: "Dasung Paperlike 13K" 13.3" with 300ppi, but it isn't exactly cheap and it still needs to be connected to a computer (which actually is a good thing as it means running your own software is much easier). So that + some small computing device and a nice keyboard would do it for a portable typewriter.
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