iMovie - freeing up space
Please do not delete or move iMovie projects or events within the Finder. If you do you'll probably end up with a mess by breaking the links between your media and iMovie projects.

Your best approach is to delete any media you're not using from within iMovie itself. 

Using iMovie to delete media is the safest way to preserve your projects as you can visually see if what you're deleting is being used in your project. In fact, if what you’re trying to delete is being used in a project, iMovie will prompt you to remove the clip from the project before deleting it. This safeguard obviously isn't available if you try to delete media manually from within the Finder.

The process for deleting media within iMovie will depend on the version of iMovie you're using. In some versions you have a Move to Trash option, while in others it's labeled as Delete Media from Event.

You may find iMovie for Mac: Organize media and events of interest. Alternatively, you can access the iMovie for Mac guide.

If the above links aren't useful because you're using a different version of iMovie, post a comment to advise what version of iMovie you're using and I can provide some steps for you to follow. Alternatively, now that you know it's best to delete your media directly from within iMovie, you can just use your search engine of choice to find the info you need.