In iMovie 11, how do I export at my custom resolution/aspect ratio?

I see a few posts that say it will also export at the aspect ratio you imported at, but how?

I recorded a portion of my screen with Quicktime's screen recorder, and it came to a resolution of 680x812. I imported that into iMovie, as well as a 640x812 image at the end, but when I go to export, I only get this screen, where none of them are the resolution of my video, and it puts black letterboxing on the sides:



How do I do it?

Also, how do I get it to not degrade video quality?