How to crop section of video and make it the video size

I want to crop my video to cut off some stuff on the outside and then have Final Cut make that crop result the new video size (smaller video dimensions than original). When I crop traditionally it deletes the content outside the crop, but then leaves black space and keeps the original video size not the crop.

There are three crop functions: Trim, Crip, and Ken Burns. It sounds as if you want the middle one. Crop reduces the size of the image proportionate to the frame size.