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.

There is almost no phones with camera that works on linux. Your best bet is phone that was specifically made for linux like pinephone or something.

I think this is the decisive list:

https://wiki.postmarketos.org/wiki/Devices

Maybe have a look at availability and how old the chipset is. These are mostly old and slow phones. It also doesn't help if RAM/memory is scarce. I don't have a good recommendation of a decent modern phone.

That's a tough one.

1. Not that many compact phones right now in general. Sub-6" is a dying breed and anything sub-5.5" is almost gone. Best bet would be some older phones but they'll all struggle with the modern web and android apps.

2. A solid camera narrows your choice down to Ubuntu Touch. Ubuntu Touch uses a compatibility layer for accessing Android drivers and firmware which PostmarketOS strongly discourages, so the absolute majority of pmOS ports have no working camera at all due to a lot of technical difficulties.

So for now look at what supported devices UBPorts has to offer.

@markkdark also waiting for a decent phone size to use with Linux myself.

Currently, I use a Fairphone 5 only because size is its only negative aspect. Apart from size, it has upgradeable battery and storage, decent performance and very good camera. As soon as there is another option at around 6 inch ready for Linux, I will sell this tablet from Fairphone.