I did some research. Far too much research, actually: Taking multiple samples per world, making sure that the client wasn't caching any additional chunks, et cetera.

• Main
  • Rising: 9x9
  • Legacy: 9x9
  • Mining: 9x9
  • SG: Unknown. IIRC SG worlds had a higher setting than the main worlds at some point, but I don't know whether this is still the case.
  • Nether: 9x9
    
  • End: Unknown. Due to the topology of the world and the absence of /fly I wasn't able to properly establish the exact value. I'd assume it's 9x9 just like in the Nether.
• VMR: 9x9
  
• Skyblock: 9x9

The TL;DR:
9x9, SG may differ.