I guess an important question is how small is unusable for you? That's not an objective measure, and will be up to what you find usable.

No experience myself, but I saw this recently on Lemmy: GPD Win Max 2

This depends on how big hands you have.

A fully usable system good enough to watch Youtube, do spreadsheets and play Minecraft can be a few inches wide.

So basically what is the smallest keyboard and screen you find usable? There's likely a laptop around that size.

I've had many laptops over the years, from the original eeePC to 17" portable workstations, and the smallest I personally found to be "usable" on a daily basis were in the 12" class; I used a Sony Z505 throughout law school. Get that size with a usable keyboard and touchpad. Anything reasonably modern with 8GB of RAM should be able to putz around in Linux as a secondary device.

A 2012 11" MacBook Air will run ZorinOS nicely and is truly tiny but very usable. Any Air made between 2012-17, really, but the 11" is SMALL.

A netbook maybe? I used to have an old 10" lenovo netbook with a celeron CPU and 2GB of RAM. Worked pretty well with Lubuntu. I could even play StarCraft on it. If you just need it for light browsing and office tools, it should work fine. You can probably get one with at least 4 or 8 GB RAM for better performance.

Is this a laptop to you? https://thepihut.com/products/3-5-ips-capacitive-touch-display-320-x-480 because the rpi runs Linux pretty well.

Not sure how much power you need - but I just bought a used Microsoft surface 7 pro and installed Linux Mint on it. Was pretty damn easy actually. Runs great!

This isn't the smallest you can go per se and isn't a laptop but I've happily run PopOS on my Surface Pro 8 for over a year. You lose the front camera due to the proprietary blobs, and there's some tweaking needed such as to get the keyboard working for LUKS decrypt but it's pretty damn good for those tasks you mention. I read a lot of comics and RPG rule/corebooks and it's perfect for that plus some light browsing and media consumption. Biggest downsides are the battery life which isn't great compared to on Windows (around 5hrs or so, up to 7 with light usage) but that's Linux for you, and ofc it's more awkward to balance the folio keyboard on your lap. But for all that, it's pretty convenient to take around plus you get the benefit of that bright, Surface Pro display

I have a Toshiba Satellite T110, 11.6" screen, now running Linux Zorin. I've had it for 15 years - got a new battery at one point and added RAM, very easy to do. It's been a cracking little machine, really nice for travelling with.

Phone or small tablet with a wireless keyboard.

How small we talking? Dell Latitude 5310 is a nice small laptop, but that might be too big. You can always get one of those GPD mini laptops.

Maybe not the smallest but I recently got a second hand Panasonic CF-RZ6 and it's incredibly useful. Surprisingly most(all?) hardware's working on Linux/FreeBSD/Win10/MacOS 10.13(OpenCore) that I tested, and the battery lasts about 4~5 hours with low clock speed on Linux. S3 sleep is also working pretty well. The japanese keyboard is hard to type on, and it's pretty hard to obtain outside japan though.

Librem 5 on its own, and with the optional lapdock for serious outdoor work?

I have an HP Chromebook 11 that I was going to install linux on (iirc you have to open it up and flip a hardware switch to do so) but I ended up keeping Chrome OS on it. Anyway I mention it bc the form-factor is nice, it's very small and light but still usable. Only thing I don't like is the touchpad so I hook up an external mouse whenever possible. You can get those chromebooks for under $100 refurbished too.

If you already have a larger Linux laptop that you're otherwise happy with, have you considered just throwing it in a padded laptop backpack?

I have an XPS 13 9310 with the OLED display it's quite a bit smaller than my M1 MacBook Air and I picked it up for ~250

https://x-plus.store/products/n150-netbook

I saw a post on this a few weeks back and excited purchased one. I've had it for a bit now and I'm generally happy with it.

If you've ever bought a Chinese product like this before, you know generally what to expect: about 95% quality and 5% WTF.

Personally I put Arch on it using KDE Plasma/Wayland and touch is lackluster. Other distros might handle things better, but I'm an Arch guy and I'm sticking it out.

• Keyboard is better than expected, but still a little janky. Key feel is surprisingly good but far from great, although sometimes they don't actuate. I think that's because I'm still learning to type on it. Key arrangement is not as big of an issue as I thought, although stuff like Tab, -, ", / can be a little awkward for typing terminal commands, plain text typing (like note taking) I can get pretty up to speed. Honestly the jankiest key is . but it's placement in the center of the cluster still makes it fairly easy to hit
• The screen is clearly a tablet turned sideways. I've seen this before and I think even the Steam Deck does this, but it does lead to some oddities like resolution being 1200x1920 and SDDM is sideways (I tried fixing it, I'm sure there's a way but I broke it so bad on one go that I ended up just doing a reinstall)
• It's hefty, feels like a solid device, although maybe even a little too hefty when using it folded over and trying to hold it with one hand while reading

For me it's absolutely perfect for the kind of note taking, book/comic reading, emulator playing, internet browsing I need to do. Admittedly it may still be too close to that "toy" kinda feel though ...

ETA Prime reviews a lot of tiny laptops. They might help you find a good match.

Possibly this. https://liberux.net/

IBM Thinkpad 701.