To preview the clip(s) on the Timeline, click the Play button in the Preview Window controls.For clips on the Timeline, the Preview Window displays a preview of how the final video will look produced with the current dimensions selected in the Project Settings dialog box.
100% Scale: Displays the clips at the actual dimensions selected in the Project Settings dialog box. If the dimensions are larger than the Preview Window, a scroll bar appears on the side and/or bottom of the Preview Window.
Shrink to fit (default): Shrinks the video preview to display the entire video within the dimensions of the Preview Window.
Project Settings: Opens the Project Settings dialog box to change the preset or custom dimensions selected.