The stripes just show you that something is there, outside of what will be shown on-stream. It's informational for you as a streamer, your viewers won't see it. You CAN turn that off in the settings or View menu somewhere if you don't care about it, but it does help in aligning and scaling sources at times (so you know if stuff IS outside the recording/streaming visible area).
If you mean the aspect ratio, it's in Settings->Video, Canvas Size and Output Size. Normally you'll want a 16:9 aspect ratio, like 1920x1080.