Hi, Booberoi,
To review some basics, creating a new project and using the already imported media will not use up much additional space. The project does not make a duplicate copy of the imported media, but rather contains thumbnails that consume virtually no space, and which refer to the original media that you imported. The original media is located in the Original Media folder that in turn is located in the iMovie library. The only way that creating a new project would consume significant new space is if you imported new media into iMovie for the purpose of creating the project. So, if you intend to drag in new .mov files, then that would take up additional space on your drive. But if the .mov files had already been imported, then using them in a new project will not consume significant additional space. However, if you want to delete the media that you have imported into iMovie, then you can use one of the methods below.
Deleting an iMovie Event that contains media will delete all the media in it. Select the Event and do a File/Delete Event. You must delete the event itself, not just the media contained in the event in order to free up space.
Another way is to select a clip and then do a File/Reveal in Finder. A folder will pop up entitled Original Media. It is a Finder folder. All clips deleted from the Original Media folder will be deleted from the iMovie library.
Be aware that deleting media from the iMovie library by either method above will delete it from all projects that may be using that media.
An easier way might be to movie your iMovie Library to an external drive with sufficient space to contain your media and new media added for new projects. First do a File/Consolidate Media to make sure that all the media goes with the library and you have no broken links. The external drive must be formatted Mac OS Extended (Journaled). Be aware that if you reformat a drive that will delete all media that is stored on it, so first back up any data that you want to keep.
Deleting media from your iMovie library will not affect your photo albums and videos that are stored in the Photos program. Photos has its own library.
-- Rich