Sorry for taking so long to respond; I had to ask one of our engineers about this. Just so I don't distort what he said... his words:
So it sounds like what she creates in desktop Photoshop is a 21600x14400 pixel canvas. This is, to my knowledge, impossible in any iPad drawing app (Procreate can get up to 10000 x 6666 though). Our app currently doesn’t allow setting canvas size at all (the canvas size on iPad Pro is 2732 x 2048).
If she exports to Photoshop CC and ups the resolution, she's still unlikely to gain the additional fidelity that she’s looking for. Currently, though, she can use Send to Illustrator CC and have Illustrator up the size and it will re-render the strokes at the higher res, (likely getting her what she wants). That feature will be going away though once we move to Layers in Sketch.
Hope that helps.
I need this too. I Need to create graphics and textures for branding that could range and will need to be printed at very large scales. It would be great to have larger canvas sizes and resolutions.
Larger canvas sizes are coming. I think early next year. Hang in there.