Hi,
The reason that you are having trouble is that iMovie 10.x.x only exports in 16:9 aspect ratio. There are no aspect ratio adjustments.
I am not familiar with Facebook's cover video requirements regarding aspect ratio.
A possible workaround would be to export your video to your desktop in 16:9 and then open it in Quicktime Player and crop the video there. The steps are:
1. Open video (but do not play it) in Quicktime.
2. In Quicktime, do File/New Screen Recording.
3. In the pop-up box, click on the red recording dot.
4. Put your cursor on your video, press down and drag-select the area
that you want displayed, in the estimated aspect ratio you want.
5. Click on "start recording".
6. In your movie clip, click the triangle arrow in the control tool to start playing the vid. The movie will play and be screen recorded at the same time.
7. When done, click on the little square stop emblem in the tool bar at the top of your screen.
8. Save.
Also, if you have iMovie 9 on your computer that still works properly with the operating system you are using, I believe that it has an aspect adjusting feature. Not entirely sure about that. But you can check and see if you movie's aspect ration can be adjusted there.
Hope this helps.
-- Rich