Emacs

Check mini PCs made by GPD.

Port emacs to collapseos and the world is your playground.

It would require more than a little work, but I'd be willing to bet that it's possible to get it to run on a microcontroller with some external RAM and an SD card attached to it. The work comes from the fact that you won't really have a full OS in the sense that you'd prefer.

Pine phone + Keyboard Case?

Product - Shenzhen GPD Technology Co., Ltd. GPD?

And I agree with using a shell version(or ssh) on Android/iPad maybe a more reasonable way.

Haven't tried it, but you could use a smartphone + Corne Choc + trackpoint

It's wired. There are versions with Bluetooth but no trackpoint.

It fits in a external hard disk case

I would be worried about battery life if you don't something that runs android.

uconsole, but the waiting time is somewhat Loooong now....

I have a Lenovo Chromebook Duet 2 and I haven't regretted it at all. Graphical Emacs is important for me as I want to view my notes on it and I cant do that well with the terminal version. So that removed any Android/Apple tablet. Unfortunate as it may be, I don't think there is any mature, budget Linux tablet and Chromebooks, which support Linux out of the box are the closest to that.

It's lightweight, with a 10 inch screen and I can comfortably use Emacs with the keyboard it comes with. You could go even smaller screen, but imo, after one point it stops being comfortable. So this is a very fine solution for me

Any modern phone + a bluetooth keyboard

Modular, you can pick whichever keyboard you want and choose how you want to trade off size & usability. There are some foldable keyboards that pack down pretty small.

Ive considered this question too. But I would opt for something other than a tablet. I want to type on the train. And anything lesser than a small laptop Im saying no. Maybe I have no experience, but tablet with no hinge on my lap, noooooo. Im already typing on my BT keyboard with my mobile, and its not ideal. Still havent bought the Thinkpad x280 that I planned.

I bought one of those 7 inch mini laptops on Ali Express. I'm a touch typist, and I can't touch type on its keyboard, but I can type pretty fast while looking at the keyboard. Some of the keys are in unusual places, those could be remapped if you have more ambition than me. I find it perfectly adequate for emacs, web browsing, email, even gimp and libreoffice on the road. I wouldn't want to write more than a page or two on that keyboard.

Not sure if Bryan Lunduke's old videos are still up on YouTube or if he pay walled them, but he used to talk about these things a lot. Small Linux handhelds with physical keyboard.

Of course, "comfortably" is another matter...

Add a few more inches, add a lot more bucks, meet the x1 Nano, have a ton more power, have a ton less rabbit holes.

I want to use this setup. https://words.kitchen.io/2021/03/13/ipad-as-desktop-emacs/comment-page-1/?unapproved=15483&moderation-hash=096a08e1351d54ae6a75ce6b2407460d

I don't know if you mean smallest, or cheapest, but my Galaxy S23 runs emacs

A Steamdeck + keyboard plenty of power