This may be the simplest question because I am very new in video editing. Basically I have a video of 640x1080 dimension (I recorded iPhone 6 screen with QuickTime). To I just want to add some text and sound to it. I know how to do the jobs; however, the problem is, when I created a new project in iMovie, I can only choose project of size 4:3 or 16:9. After importing, editing my video, and exporting, I will have a video with black borders at 2 sides. How can I preserve the original dimension?