You need to make sure first that all the media is in your library and it does not contain links to external media that will break when you move your library. You do this by choosing selecting the library in the libraries pane and choosing File - Consolidate Library Media.
If your library is on an external drive you can simply reconnect that drive to the other mac; otherwise you will have to copy the library package (it looks like a file with the extension .imovielibrary) to the other mac. Launch iMovie 10. The first time iMovie won't know where the library is so you will have to choose File - Open Library - Other and navigate to the library location.
That's a very average size. It contains not only copies of your original media but all the transcoded media as well (which is even bigger).
If your library contains other projects and events that you do not want to work with on on your second machine you could make it smaller by creating a new library containing only the project you are working on and the events that contain the media it needs. (I am not sure whether you want to permanently change machines or be able to switch back and forth.)