To make working on a mobile design easier, the editor can display guides that indicate the size of the device screen. This allows you to place components at an appropriate location on the canvas, prior to viewing your Design on your mobile device. To display screen guides, choose the appropriate item from the Screen Guides sub-menu of the View menu.
Once you have chosen your screen guides, red lines will be drawn on the canvas to identify the screen edges. If you choose guide dimensions that correspond to a mobile device, you will be prompted to format the Design for mobile if you have not already done so. If your design does not have any components yet, the blank-canvas message will be displayed in the center of the device screen region, as in this picture: