You can change the zoom in the viewer using one of the following commands:
You can also use two automatic zoom modes: Fit in Window (the F5 key) and Fit Width (the F6 key). The first calculates the zoom percentage to show the whole image in the viewer window. The second shows the whole width of the image in the viewer window (it avoids horizontal scrolling, but vertical scrolling can still be needed). You can set one of these modes as default (then it is used every time you open the viewer); see Appearance tab in Configuration.
You may also want to view the image at 100% zoom (one pixel of the image is one pixel in the viewer window). You can use the Actual Size command (the F7 key) for this zoom.
You can find all these commands in the viewer's View menu.
The Zoom mode in Using Mouse in Viewer.