Maximum iMovie Library file size

I don't think there is any limit but it would be wise to create new libraries rather than letting one get too big . . . all your eggs in one basket?

See this AI generated response covering the advantages of using multiple libraries:-

Yes, you can have multiple iMovie libraries on your Mac. 

Multiple libraries allow you to organize your projects and clips more efficiently. You can create additional libraries to:

• Archive older projects
• Free up storage space by moving events to external drives
• Work on a project on a different Mac
• Avoid performance issues caused by large libraries with many projects 

To create a new library in iMovie:

1. In the iMovie app, go to File > Open Library > New. 
2. Name your new library and choose a location (e.g., an external drive or a specific folder).
3. Click Save. A new, empty library is created with a new event. 

You can have multiple libraries open at once. To switch between them:

• Use the Library dropdown menu in the top-center of the iMovie window. 
• Or, go to File > Open Library and choose from recently opened libraries, locate a library, or create a new one. 

You can also move projects, events, or clips between libraries using drag-and-drop (move) or Option-drag (copy). 

 This lets you reorganize your workflow while keeping projects self-contained and easy to back up or share. 

I suggest that creating a unique (individual) iMovie Library for each Project is the best way to manage your media and projects. It minimizes Library size and is also a clean collection of the media associated with each project.

An iMovie Library where you continue to import more & more media & create more & more projects becomes increasingly complex for management of the Library as well as becoming increasingly risky for the retention & backup of all your media.