When you open Narration Editor, the timeline will be scaled so that all slides fit it without scrolling. That's a convenient starting point, but when you edit the timeline, you may need to zoom to a specific part of the presentation.
To scale a timeline:
Use the Plus and Minus buttons in the bottom right corner of the Narration Editor window to adjust the scale.
There is also a quick way to scale the timeline to maximize a selected slide and to zoom out to show the entire presentation.
To zoom in to a selected slide:
To zoom out to show the entire presentation: