If you set the canvas size to the size of your recording, you will not get black bars. There is a "Fit to Visible" option in the Project Settings dialog. This will resize the canvas to the current content on the screen.
The one exception is if you are sharing to YouTube. If the aspect ratio of your recording is not 16:9, then you will see black bars. There is no great solution to that. One option is to fill up the extra space with a logo or something.