Is there a keyboard shortcut to change the brush size in the sprite editors? I noticed quite a few others missing actually that would be very useful.

you can check some of the shortcuts here: https://neko250.github.io/pico8-api/ in the keys section, i don't know if it's possible to control the size via a shortcut atm though,

the keyboard shortcut i'm missing the most is selecting notes (to copy/paste them),