You can only resize the draw.io diagram viewer in the editor at the moment. By changing the zoom on the diagram, you can make it appear larger or smaller on the Confluence Cloud page. Note that this is based on the original diagram's dimensions, and not on the default draw.io macro size.
- Edit your diagram and make sure you have selected nothing (click in a clear area on the drawing canvas).
- In the format panel on the right, click Zoom and change the zoom percentage.
- Save the diagram and exit the page editor to see your changes.