Scale
You will find this button within the extension of 
under 
The Scale function allows to enter a zoom-factor for the window. 100% means that 1 cm PCB = 1 cm on the screen = world coordinates. You can set units inch, mil and mm (1 inch = 1 = 25,4mm; 1 mil = 1/1000 = 0,0254mm). To allow a display of world coordinates, you have to enter the exact usable size of your screen in menu Settings - Settings/Options.
Image: How to set the usable monitor size