How to change aspect ratio in IMovie?

After clicking the blue reset button in the upper right to apply the crop, you then drag your clip into the project. The procedure as I described it, using the Project Media view, assumes that you already had the uncropped clip in your project so that it appears in the Project Media view. You can also do the crop procedure in the Library media browser view.

-- Rich

Yes, you can get a banner look, but it will require cropping. Here's the help link that explains how to crop a clip in the browser:

https://help.apple.com/imovie/mac/10.1/#/mova56757e7e

So, you can open a project and In the Project Media view you select the clip you want to crop. Then click on the crop icon (the one that looks like intersecting angles) in the tool bar at the top of your screen. Then click on the Crop button that appears to the left above the preview screen. Adjustment rectangles will appear that let you adjust to any dimension that you want. Then click on the blue Reset button on the upper right of the screen to set the crop. The sequence looks like this:

-- Rich

The first clip in a movie sets the aspect ratio. You are free to create a new project, put a clip with the correct ratio on the timeline and continue from there.

You are welcome. :)

-- Rich