How to change the resolution of projects and compound clips?

I have a large series of projects and compound clips within Final Cut Pro v 10.5.2. I would like to change the resolution because I originally set it to 1250 x 720 but I only need 903 x 720 (I have black bars on either side of the image at the moment and it's a bit pointless for me to leave them there).

It is the compound clips that I am exporting as video, not the projects. It's a workflow I've had to adopt because I have been cutting up very long VHS tapes and separating them by sequence.

I notice on the compound clip's information panels that the resolution is greyed out, so I can't click on it. I tried instead changing the resolution inside the Project's information pane. It changed the resolution of the project to 903 x 720 but the individual compound clips remained the same (1280 x 720).

Perhaps there is another way of changing the resolution that I'm not aware of?