You asked what “size” the iMovie background images were in. They are in an aspect ratio of 16:9 (ratio of width to height). As near as I can tell from the project settings box, they are 1080p in size, which means 1920 pixels x 1080 pixels. You can choose a different resolution (“size”) when you share out your movie. For example, when you can choose 1080p or 720p resolution when you share it out, and they both will be in a 16:9 aspect ratio. So, if you use your own background image just choose any resolution that is in a 16:9 aspect ratio because iMovie is going to render it at 16:9, and if you chose a different ratio you will get black bars. Since iMovie uses 1080p (1920 x 1080) you might as well stick with that if you plan on sharing out at 1080p (high def). Below are other resolutions ("sizes") that are in a 16:9 aspect ratio:
16:9 aspect ratio resolutions: 1024×576, 1152×648, 1280×720 (720p), 1366×768, 1600×900, 1920×1080 (1080p), 2560×1440 and 3840×2160 (4K). So, if you plan on sharing out at 1080p, choose 1920 x 1080 size. If you plan on sharing out at 720p, choose 1280 x 720 size.
I hope that the above is anywhere near clear.
Best,
Rich
