From Showit Manual
The stage is the main area in Showit that allows you to drag Layers on and layout the look of your site. The stage currently works off of a 1280 width and 865 height as a standard aspect ratio to use for the majority of web browsers.
However keep in mind that although the aspect ratio is fixed while designing, it is important to understand the Site Layout used once you publish a site. It is important to Preview your site at different aspect ratios even though during designing you will only see the standard design ratio.
You can Zoom in on the stage to see details and make fine tune adjustments and also view a Grid on the stage to help line Layers up. It is also recommended that you use the Alignment tools to easily align and distribute groups of layers.