Hi, andrea,
Here is the online help item that explains how to delete a project:
https://help.apple.com/imovie/mac/10.1/#/mov79fa23112
However, deleting a project will not delete the media that it contains. It puts the media into an event that bears the same title as the project. To free up space you would then need to delete the Event (the Event itself, not just that media in it) that contains the media. Deleting the Event will then delete the media from the deleted project and also from all other projects that are using the same media. So you need to be careful.
Here's a help link that explains how to delete events and the media that they contain:
https://help.apple.com/imovie/mac/10.1/#/mov74d7d370c
A few practice notes:
Before you transfer a library to an external drive, it is best to first do a File/Consolidate Library Media to insure that all media that is contained in the library remains with it when transferred. Usually there is nothing to be consolidated, but sometimes there is.
Also, the external drive should be formatted Mac OS Extended (Journaled) in order to store an iMovie library. With later operating systems you can also use Apple's APFS format. Be aware that if you need to reformat the external drive the reformatting process will erase all data on the drive, so first back up any data that you want to keep.
Before moving anything, it is good practice to do a Time Machine backup in case something goes wrong.
As a final note, the safest procedure is first to copy (not move) your library over to the external drive. Then, after you have confirmed that the copied library is working properly, you can delete the original library on your internal drive if you want to do that. You should always have more than one backup of your library, as well, because libraries can become corrupted and you will then lose the projects and media that are stored in it.
-- Rich