Oddly enough, my purchase decision was driven
specifically by considerations of size, resolution, and suitedness to task.
Again: at 8", e-ink is pretty broadly useful. If you're frequently reading scanned-in journal articles, the 10" or 13" devices shine, though these can be accessed on smaller screens using in-page zoom-and-scroll. (Onyx BOOX has several settings for this in its NeoReader app.)
Note-taking, which was not a use I anticipated using, also happens to be really well-suited.
Yes, you can read on a smaller device if you must. However you're making the same sacrifices for mobility that are present in pocket-sized printed books, and the format is best suited to largely unformatted text (e.g., prose). Diagrams, tables, and other layout translate quite poorly, and this is intrinsic to the display itself.
The one task to which the tablet format seems best suited is precisely e-book reading. So I've ditched the "smartphone" (a pocket snoop) and settled on laptop / desktop (productivity) + tablet (ebooks), and dedicated devices for specific other applications, most especially capture (audio, images, video).
"The Case Against Tablets"
https://joindiaspora.com/posts/880e5c403edb013918e1002590d8e...