The EditLive! toolbar provides the following buttons button for selecting Page Widths:
ThePage Width item is located under the View menu.
Choosing Page Width
Both Toolbar button and Menu item display a drop down menu of common display widths for Smartphone, Tablet and Monitor as well as Sizing to fit the editor and a Custom width.
Size to Fit
Choosing Size to Fit displays the content of the editor at the width of the editor on the screen.
To display the editor content for a custom width, choose Custom... from the Page Width drop down. A dialog is displayed to enter a custom width in pixels.