Click on the page.
Hit Ctrl and 0 ( zero ) for default size
Zoom in - Hold down Ctrl and use + in stages
Zoom out - Hold down Ctrl and use -
To manually control size of box
Click on top right, middle icon until just one box is shown
Drag and drop the whole page to the left by grabbing the top border
Drag and drop the borders to the middle after you hover over the border and get the 'double arrowhead effect'