The Planck (OLKB.com) is approximately 23 cm, it's already quite compact and requires a major adjustment in typing habit to get used to.
The Atreus42 is available from Falba.tech, that's about the same size but has a slightly different layout.
There are smaller boards, but I can't tell you what they're called. I'm sure others here can list names off for you.
Those are your mechanical options. There are plenty of compact keyboards made for tablets that you can consider. You can google iPad mini keyboard and you'll get a host of online links.
Good luck finding what you're after.