Canon EOS 80D Manual (en)

An overlaid image (the overlay image) can be displayed 
enlarged/reduced, rotated, or with its composite ratio adjusted 
Move the camera and subject to compose your 
You cannot use the Overlaid Display function in the [Zoom View] 
Enlarging/Reducing an Overlay Image
An image displayed as an overlay (the overlay image) can be enlarged/
reduced from 25 to 200%. Use the Enlarge/Reduce slider to set the 
enlargement or reduction.
Drag the Enlarge/Reduce slider left or right.
The enlargement/reduction set with the Enlarge/Reduce slider is 
applied to the overlaid display.
You can also set the enlargement/reduction by entering a value 
You can also move the cursor over the slider, click, and then 
press the <> or <> key on the keyboard or use the scroll-
wheel on your mouse to set the enlargement/reduction.
Clicking the [Reset] button resets the image to its default state.