Typically, a Vision Client is designed to run on multiple different resolution and sizes of monitors. The various component layout features help design elastic screens, but sometimes you need to set a lower bound as to how small you'll allow the Client's usable area to shrink. This is what the Minimum Size settings are for. You can see these settings visually represented in the Designer as lines on the Vision workspace when the Root Container is smaller than the configured Minimum Size..
Whenever the usable space shrinks smaller than these bounds, scrollbars will appear, capping the Width and Height to these minimums. This defaults to 800 x 600. In the image below, the project was set to a minimum size of 400 x 300. Since the window is smaller, the outline is visible.