How can you get clips from different YouTube videos to have the same resolution or frame size in Adobe Premier Pro CS6 2017?
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.