I don't understand the video, or it doesn't work for me.
I know I can change the height of the preview section at the top by dragging the horizontal line below it up or down, and I can drag the horizontal line between the video tracks section and the audio tracks section to change the relative heights of those sections. But that doesn't change the height of any of the tracks.
And I can't drag any of the lines between video tracks or audio tracks to change the size of those tracks.