Failure to display is not lack of hard drive space, because the Photos app vids that are displayed via the Photos item in the iMovie media sidebar are not actually in the iMovie library Original Media folder until they are imported into iMovie. If you lack sufficient hard drive space on the drive where your iMovie library is located, then that would prevent you from importing them. But you can still display them before importing.
Possibly the vids are not being displayed because you are using iCloud Photos with optimized storage enabled. With optimized storage some videos may be stored in the cloud instead of your drive, with just an editing thumbnail in the Photos app.
From the Photos app, try doing a File/Export Photo to your desktop and from there dragging the file into the timeline of an iMovie project.
To free up some space on your hard drive you can open iMovie and click on iMovie/iMovie Preferences and delete the render files from the preference box that opens. They are just temporary files that iMovie will recreate as necessary.
-- Rich