I'm not aware of any directly comparable non-DIY devices. There are a few other "electronic typewriter" devices that usually come up in these discussions, like the Alphasmart range and the Freewrite, but the DM30 checks more of my boxes.

> And is it actually that small that you can put it in your pocket?

I (a man who wears men's clothes with generally usable pockets) can fit it in a pocket, but not very comfortably. It has a footprint similar to a basic e-reader, but it's thicker, I'd guess an inch and change.

> Which document formats does it save to?

It just saves to plain text, which is perfect for me since I write in Markdown. The editor uses a (sort of ugly) monospace font and has some simple but useful navigation commands.