when I export my movie from iMovie, the video is super small with a large white border

I suspect that you might have imported a thumbnail into iMovie rather than the full sized file. As Ian says, check out the properties, including file size, of one of the clips.

Do a test by creating a new project and importing a full sized file into it, and then exporting the project to your desktop. When you play the exported video is it full size?

-- Rich

Give full details of your media, the project properties and how you are exporting.