Is there a max resolution for images in an sbix font?

I put an iColor 1024 layer and noticed that on export it was downsized quite a lot:

on Export:

What’s the maximum size for images in an sBix font?

That’s what the renderer makes of it. You’ll have little control over that. A renderer will likely scan and prepare the pictures at an optimized size, so it can go for performance.

I don’t know if there is a limit. But 1000 px should be fine.