Unwanted Letterbox Effect in Full Screen Mode - Working with 8K Clips

The export, by default, will be in the same size as your project.

By the way, you don’t have to make your project be 8K. Your mac screen does not have that many pixels, anyway. The biggest advantage of filming in such a large frame size is to be able to scale up if necessary. For example, if you put an 8K clip in a 4K project you can then scale to 200% with “real pixels”.

Regarding “letterboxing”: when you play a clip in full screen, it is scaled to fig the screen the best way possible. To give a more striking example: suppose your video is square, say 3840x3840, and your screen is 1600x1000. Your video will have to be scaled to fit the screen, at 1000x1000. In this case, since the screen is wider, you will have black on the sides; it is not an added effect, it is just that the video does not completely fill the screen.

Most mac screens have a 16:10 aspect ratio, whereas your video is 16:9. When playing fullscreen there will be black bars on top and bottom. This is just basic geometry.

Do this, in the Viewer, turn on the Position OSC, and that will show you the true edges. I bet outside of those true edges are just black filling up the Viewer's space, that's not inside your frame.

Another option is to go into Settings->Playback and set Player Background to Checkerboard. If you have clips not filling the frame you will see the checkerboard.