Have you taken a look at those YouTube settings again? Make sure nothing is off. I would experiment with that.
Is it at a small size or low quality because of the video clips? The aspect ratio is set by you in the editor, presumably, so the size has nothing to do with the video you import.
Open the video file you exported on your computer, not on YouTube. If it has te black bars, you exported it that way and need to change it.
Yeah, if you’re using low quality videos, it will stay low quality. You can’t increase the quality of the original clips. Make sure when you export to set the quality. Default is at 60%, I like to go 80%.
No matter what you upload to YouTube, the quality will decrease a little bit because they use lossy compression. Update us with what you find, good luck!