Crop clips in iMovie on Mac

Move and resize the frame until you’re satisfied with the result.

To apply the change, click the Apply button timeline whose aspect ratio you want to restore.

To show the cropping controls, click the Cropping button.

Click the Fit button.

The clip’s original aspect ratio is restored.

Note: If you restore a clip that has a 4:3 aspect ratio, black bars appear to the left and right of the clip.

iMovie User Guide