iMovie says there is not enough space on my Mac

First, see where the space is

Choose Apple menu → System Settings, click General, then Storage, to see how much space the Mac has left. iMovie keeps everything you import and every project in one library file, by default called iMovie Library in the Movies folder of your home folder. To see its size:

du -sh ~/Movies/iMovie\ Library* 2>/dev/null

If you have already moved the library to an external drive, what you import saves there, so check that drive's free space too.

Delete render files

1. In iMovie, choose iMovie → Settings, or press Command-Comma.
2. Next to Render Files, click Delete.

Apple's description is that this removes render files from your open libraries, freeing up storage space, and that no imported media is removed. If you delete them while a project is exporting, the export may take longer, so let an export finish first.

Delete whole events, not clips

1. In the Libraries list, select the event you no longer need.
2. Choose File → Move to Trash.
3. Empty the Trash in the Finder. Until you do, the space is not freed.

Apple is plain about this: to free storage space you must delete an entire event, and deleting clips from an event does not free any. Selecting a clip and pressing Delete only marks it as rejected, which keeps it in the library. If a clip is used in a project, iMovie asks you to remove it from the project first. Delete an event only if its footage is somewhere else too, or you are sure you do not need it.

Move the library to an external drive

The library can live on a USB or Thunderbolt drive formatted as APFS or Mac OS Extended (Journaled). Apple does not recommend SD cards, USB flash drives or shared network storage, and the library cannot go on a drive used for Time Machine. It also should not sit on a drive formatted as ExFAT, FAT32 or NTFS, in Desktop or Documents with iCloud syncing them, or in a Dropbox or Google Drive folder. Back up your Mac first, then:

1. Quit iMovie.
2. In the Finder, choose Go → Home, and open the Movies folder.
3. Drag the file named iMovie Library to the external drive.
4. When it has finished copying, double-click the library on the external drive to open it in iMovie. If iMovie asks for access to a removable volume, click OK.
5. Check that the moved library contains all your media, then drag the original in Movies to the Trash and empty the Trash.

From then on, what you import and every project save to the drive. If the drive is not connected when you open iMovie, it starts with a new, empty library in Movies; connect the drive and double-click the library to go back to it. You can also keep one library on the Mac for current work and move finished events to a library on the drive, by dragging an event from one library to another.

If the Mac itself is full

iMovie shares the disk with everything else, so the space it needs may be held by exported movies, downloads or the Photos library. See how to find large files on a Mac, the Photos library is taking up space, and your Mac storage is full: what to delete first.

Where the app comes in

RefreshYourMac has no iMovie category, and it never opens or deletes anything inside an iMovie library: render files and events are iMovie's to remove, with the steps above. What it can find is the video around it. Large Files lists files over 200 MB in Downloads, Desktop, Documents and Movies, such as movies you exported and no longer need, rated Review before deleting, and moves what you clean to the Trash. In the download edition, Move to… puts a file on another drive instead. In the Mac App Store edition, Downloads and Movies are included from the start and Desktop and Documents once you allow them in Settings → Folders.