The screen outline in the Designer is only a guide and in many cases, especially if you are using anchors extensively, you can ignore it. But if this is not convenient then you can create a Variant - click on the "Variants" tab at the bottom of the Views Tree panel and then click on "New Variant". You can select one of the pre-defined sizes or set any size that you wish. Generally speaking, though, you should avoid designing a layout for one specific screen size.