How do I zoom or resize a cover in the mockup editor?

The crop window uses the cover's displayed width and height as its zoom controls. Increase the Width or Height values to zoom in, or reduce them to zoom out. The aspect-ratio display helps you monitor the image proportions.

You can also use Fit Horizontally or Fit Vertically to preserve the cover's proportions while fitting one direction. Stretch to Fit fills the entire area but may distort the cover, so use it only when the template and cover proportions are already close.