As Air Graph's post said, you can export at a lower resolution and bit rate to make the file smaller. You can also chose faster compression and low quality. There will be a quality reduction, but for internet purposes that might not make any difference.
Will the participant's in your course need to download the video, or can they just stream it from your website? If they don't have to download it then the file size wouldn't make any difference to them. The main thing is how much space you have on your website.
One way to lower the file size is to share it out to email rather than to file. When you share to email you can choose small, medium or large resolution. Then drag the movie out of your email to your desktop.
After storing the final movie on an external drive, you can delete the project from iMovie. But then you will lose the ability to edit the project in case you want to make any changes, or export at different settings. Rather than delete the project you might want to create a new iMovie library titled "Course" and store the library on an external drive with sufficient space, and formatted Mac OS Extended (Journaled). Import your media into the library on the external drive. That way all of the original media will be in the library on the external drive and not take up space on your computer's drive. If you need to reformat, be aware that reformatting erases all data on the drive. So first back up any data that you want to keep.
Deleting a project doesn't save much space because projects only contain small sized editing thumbnails that reference the original media stored in the Original Media folder in the iMovie Library. If you delete the project and the original media, that will permanently remove the media from your hard drive and you will lose it unless you have duplicate originals stored in a Finder folder on your drive or external drive that is separate from iMovie. It is a good idea to keep your original media stored in a folder on an external drive, in addition to iMovie, rather than put all your eggs in one basket. It can be cumbersome to retrieve files out of iMovie to use for other purposes.
Here's a link that explains how to use and manage multiple iMovie libraries:
https://help.apple.com/imovie/mac/10.1/#/mov3fa25bae7
-- Rich
