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.