Sony 55.9mm x 98.9mm x 11.0mm 103 g
Fiio 53.3mm x 92.5mm x 11.5mm 83g
Wow, so it is! I'd say these are about as small as an Android player is going to get to have usable size screen & enough battery.
Only other small device I still use for Spotify offline (no SIM in it and long battery life if only used for music)) is the first gen iPhone SE 128GB (or maybe 64GB will do) 123.8mm 58.6mm 7.6mm 113g. Great sound output too really and plenty of arm-straps designed for it. Pickup cheap 2nd hand.
Or what about an iPod Touch 123mm x 58.6mm x 6.1mm 88g. Should be plenty good enough for Spotify unless you have demanding headphones.
No idea on sound quality but this really is as small as your going to get for offline Spotify with a wired cable I'd say
https://www.mightyaudio.co.uk/products/mighty-vibe
- Plays Spotify & Amazon Music
- Tiny & lightweight
- Wireless syncing
- 5+ hours of playback
- 1,000+ song storage (8GB)
- Bluetooth & wired headphones
- Drop & water resistant