On the simulator or on the Vita? The simulator has a different resolutions from the Vita. Remember that PS Suite is aimed at various phones and tablets as well as Vita, so you should try to handle different resolutions gracefully.
What niwrA says is 100% correct, you should ensure that your UI will work on any screen by having some screen elements with fluid sizes to compensate.
However, if you would like to force what size the simulator is (I tend to force mine to the PS Vita size, as that is the platform I mainly want to target!), then the simulator will pick up the first two parameters of your GraphicsConext constructor call and use that screen size. For instance, where you define / construct the GraphicsContext in your AppMain class...
Private GraphicsContext mGraphics = new GraphicsContext(960, 544, PixelFormat.Rgba, PixelFormat.Depth24, MultisampleMode.None);
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