I understand all the requirements, having OS X Yosemite, etc... but my question is how to do it with iMovie? iMovie doesn't let the user change the dimensions of the movie screen size. They said it is possible but I don't see how. If anybody has done it with iMovie, some help would be greatly appreciated!
Thanks
Here is a trick using just iMovie and Quicktime to get the right dimensions.
Now, any clips that you export will be in the right dimensions.
If you need to hide your short clip, you can shorten it to 0.1 seconds. Put a title over it and delete the text. Now your video starts with 0.1 seconds of a black screen.