When the source of the recording does not have the same width and height as the desired output format, it is inevitable that you either 1. crop the source, 2. stretch the source and loose the aspect ratio, or 3. have black bars at two sides.
If you want to adjust the size of a source, either right click and select
Transform, or use your mouse on the red bounding box. See:
OBS Studio - Free and open source software for live streaming and screen recording - obsproject/obs-studio
obsproject.com