I have a Onyx Boox M92 clone, a 9.7" e-ink e-reader from late 2011. It’s a Linux device but with proprietary software. Someone managed to put Ubuntu on it, even added a bootloader. See the amazing video on this page: https://www.mobileread.com/forums/showthread.php?t=217438
(It didn’t work for me, as I was unable to change it to boot the right partition).
As for size, I think a 10" e-reader is the right one for me. It is large enough to read A4 PDFs (especially if you crop the margins) or websites. It is not that heavy to be that tiresome holding it, and it is portable enough to take it easily outdoors.
For a smaller e-reader, ultra-portable, you could just have a Librem 5 with an e-ink screen. That would be so awesome if someone makes it!