How can you get clips from different YouTube videos to have the same resolution or frame size in Adobe Premier Pro CS6 2017? I'm new, so pardon my incorrect terminology.

Let's say that I download different YouTube clips using an online converter. When I put them into a sequence in Adobe Premier Pro, the clips from the different videos tend to have different frame sizes. I could scale to frame size or set to frame size, but this has lead to black bars on the side or top of the video. So that isn't what I'm looking for. In the end, when I upload to YouTube, I want all of the videos to be the same size so that there are not any black bars in the final product.

The second step was to create a new sequence first and have that start off the frame size I'm looking for. So, for example, I want to upload to YouTube, so I use 23.976 frames, 1920x1080, and other basic settings for the sequence. However, when I upload my clips after that to the sequence, the clip is tiny compared to the program monitor's preview display, and when I add different clips from different vids, I still have the same issue that they are not the same frame size. Is there a way to fix that?

So, I hope I was clear enough. If anyone needs any further clarification, please let me know and I'll try to answer it. I'm new to this stuff, but I'll do my best.
There is no ideal solution to your problem, if you don't want the black bars.

Pick your main timeline frame size and aspect ratio, and make all the clips fit that timeline. This, of course will reduce the resolution of your smaller clips.

If most of your clips have a frame size of 1280x720 and therefore a 16x9 aspect ratio, then create a timeline that has that frame size and aspect ratio. If most your clips have a frame size of 640x480, then that's your frame size, etc.

Then simply fit the clips that don't have your chosen frame size and aspect ratio to your main timeline by adjusting their scale in a way that any black area is pushed off-frame.