Deleting movies from iMovie

To create space it is best to invest in an external drive and move the iMovie library to it. (Control-click on the library's icon in the Movies folder, then do Edit/Copy, and then Edit/Paste it into the external drive. Then, after confirming that the copy is working, you can delete the library from your internal drive.) Before copying or moving the library, make sure that the external drive is formatted Mac OS Extended (Journaled) or APFS. If you need to reformat the external drive, be aware that reformatting erases all data on the drive, so back up or remove elsewhere any data that you want to keep.

For information, the movies (projects) in the iMovie app itself take up very little space. The projects contain only editing thumbnails, that you see in the timeline. iMovie refers to the full sized media that is stored in the iMovie library's Original Media folder for the project. Therefore, deleting the project won't free up much space. To free up space on your drive you would need also to delete the project's media from the iMovie library. (See below.) When media is deleted it is removed from all projects that may be referring to that media. So, though you may not care about deleting an unwanted project, it is possible that some of the media contained in it might also be contained in or referred to by other projects, and by deleting it from the unwanted project you also would be deleting it from projects that you wanted to keep. Also, unless you had the media backed up outside of iMovie you might permanently lose your media.

If you wish to delete a project to save space, despite the risks above risks associated with doing that, you would need to go to the project browser screen (where your projects are displayed as icons) and move your cursor over the project's icon. That will reveal a little circle with dots in it to the right of the project's title. Click on the circle and then click on Delete Project from the drop down menu. The project will then be deleted, and its media will be placed in an Event of the same name as the project. To save space you would need next to delete the Event itself, and that would result in deleting all of the media in it.

-- Rich