It'll be coming in the next update, but for the time being, just set your image.__private__.xscale and yscale to the scale you want.
Thanks! It actually ended up randomly working after a bit.
But it made me have another question : so I have this sprite of a button that's from my figma design, it has a precise size and all. I would like to create a button that uses this sprite (without nine slice), with hovering and all, but AutoUI seems to automatically resize it. Is there a way I can disable this automatic resizing for buttons?