Okay, what resolution is your monitor, and what base resolution are you streaming at? If your monitor resolution is much lower than the standard 1920x1080, and you're trying to use 1920x1080 as your base resolution, then you need to either resize the sources (using edit scene button and resizing them yourself), or resize the base resolution.
You just generally don't seem to have your sources set up and sizes are just mismatching. Get the sizes matching, fill in the black sides however you want, and you should be good to go.