Click anywhere on the layout background and use the "+" (in) and "–" (out) buttons to zoom the entire layout, or use the mouse wheel to zoom the layout in and out centered on the cursor location.
Fit in View
Fit In View scales the Viewing Grid so that all items in the layout are visible. It is a convenient way to restore a layout you have zoomed or moved.
Right-click on the layout background to open the context menu and select Fit in View.
Fit In View is performed automatically when you change to Fullscreen Mode (see "Expanding Items to Fullscreen Mode").