Using the Zoom tool
The Zoom tool in the Score Editor works much like in the Project window:
Click once with the Zoom tool to zoom in one step.
Hold down Alt and click once with the Zoom tool to zoom out one step.
Drag a rectangle with the Zoom tool to set a custom zoom factor.
The section encompassed by the rectangle is zoomed to fill the window.
Hold down a modifier key and right-click with the Zoom tool to open the Zoom context menu, and select the desired Zoom setting.