Set preferences in iMovie on Mac
You can customize iMovie to fit your work style using iMovie preferences. To open iMovie preferences, choose iMovie > Preferences, or press Command-Comma (,).
Apply slow motion automatically: Select to automatically apply a slow-motion effect to clips imported into iMovie that were shot at a frame rate higher than 30 frames per second (fps).
Photo Placement: Choose the default behavior for photos added to a timeline in iMovie.
Fit: Choose this option to automatically resize photos to fit within the aspect ratio of your movie. For more information, see Crop clips in iMovie on Mac.
Crop to Fill: Choose this option to resize a cropped portion of your photos to fill the entire frame of your movie.
Ken Burns: Choose this option to automatically apply a zoom effect to photos added to your movie. For more information, see Add the Ken Burns effect in iMovie on Mac.
Photo Duration: Enter the default duration (in seconds) of still photos added to the timeline. For more information, see Add clips to your movie in iMovie on Mac.
Transitions: Enter the default duration (in seconds) of transitions applied to clips in the timeline.
Delete: Click to remove render files from your open libraries, freeing up storage space. No imported media is removed.
Important: If you delete render files while a project is being exported, the export may take longer.