I want to buy a mobile phone that would be great for linux mobile (ubuntu touch or postmarket os), fairphone 4 would be almost ideal if it wasn't so big (i'm currently using pixel 8 + GrapheneOS). What do you recommend for my desire to use a compact mobile for linux + a solid camera? Thanks for the answers.
A Pixel 3a may be a good choice. It's older, but not huge—and it's very well-supported in Ubuntu Touch (and Droidian, both use Halium/libhybris to re-use the Android kernel drivers), and also in postmarketOS (mainline Linux 6.9.3 as of this message).
On postmarketOS, camera support is not fully there—the front camera is somewhat supported. Also, Wi-Fi is still a bit annoying, calls only work with headset on postmarketOS, so I would say: Use Ubuntu Touch or Droidian for now, and maybe move on to postmarketOS once it's a bit more solid.