While we do not currently have native options for editing images in Unstack and highly suggest services like Canva, Sketch, Figma, Photoshop, etc. there are ways to change the width and height of a media within Unstack page editor.
How do you do it?
The width can be controlled by Max-width within the element's toolbar:
When this input is left black, it'll go auto, or you can set this to either make the image larger or smaller in width and height will proportionally follow.
Or, you can use padding to adjust the positioning via the padding option