How large? If you’re wanting something handheld, then an old phone could suffice assuming it runs a compatible OS.

If the phone doesn’t support wireless charging you could either add a Qi receiver to it or get a dock into which it could plug.

Apple devices you could send the controller playing screen to an AppleTV and then to your TV via Airplay.

Apple TV again, Tunesmap (app) gives basic control and what’s playing.

As you say, a big cheap tablet but with an Apple device there are lots of wall mount solutions. Sonos and Google and not talking much and lawsuits both ways.

There is a user on here called PWT, he would know if the SoCo “system” could output artwork using a Pi. I use SoCo but only very basic as my Python is not up to much.

RIP the Sonos CR100 controller although the screen was small

Larger than handheld, I don't want to touch it other than turn it on when I start listening. I'd just like something that's big enough to show the artwork at a decent size, maybe 8-10 inches.

A tablet then. I’ve had various devices for ‘display’ purposes in the kitchen, most of which are no longer supported now as the OSes are too old. I’m presently using an old Nexus 5X in a charging cradle, but that would be too small for your purposes.

I would look into the latest Kindle Fire or a refurb iPad Mini. 

I’m going to repurpose an old Nexus 9 for this then. It’ll mean buying a new tablet to browse for tracks on - does anyone know if, when browsing a local library in portrait mode on an 8” tablet, albums will be 3 to a row or, 4 to a row? It’s an obscure question, I realise, I get 2 on a 6” phone and 4 on a 10” tablet, but would like 4 on a 8” tablet too...

I use an generation 1 iPad running Sonopad for full screen portrait mode "Now Playing". Still works with S2. Basic stop, start and next track works. Sonos have missed a trick not having this kind of display. In my opinion it would be extremely useful to have a basic controller available with simple controls available for much older Android ans IOS devices.

Echo Show 5/8 work very well for me, allowing the Sonos controller to be bypassed except for making/dissolving groups. This needs Sonos kit with line in jacks to which the Shows can be wired. And used as a front end device using voice commands or with casting from the phone also possible for Amazon Music and Spotify.

I also have a USB stick with some 800 albums on it, plugged into a Raspberry PI, with an instance of My Media for Alexa on it. Echos can then do voice commanded play of songs/albums/playlists from it, with album art, served wirelessly from the Raspberry.