I regret not posting it before Canvas 2025, but hopefully it'll be useful for people playing it in 2026. All letters are 5 pixels tall, and most 3 pixels wide (some 4, a few 5). I've also included a few Cyrillic letters and the digits.
I tried to make it even smaller, but it gets really funky.
Ah, that's OK.
As I mentioned in another comment, I'm aware this is not the "only" way to do things. It's more like a starting point - 5x3 letters look decent enough, they're easy to distinguish, and even for the same size you can twist things up a bit (like adding a few pixels to ⟨A⟩, or making the ⟨8⟩ only seven pixels in total).
For example if you have less space you could make most letters 2x3 or 3x2, but it'll look messy.