Set the size of the XNA game window

in window mode, set the properties PreferredBackBufferWidth and PreferredBackBufferHeight to set the width and height of the XNA game window.

public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = "Content";
graphics.PreferredBackBufferWidth = 1024;
graphics.PreferredBackBufferHeight = 768;
}

Exit XNA game in fullscreen mode

if there is no exit button or no other way to exit, just press Alt+F4

Run XNA in fullscreen mode

in Game1() constructor set the property IsFullScreen = true;

public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = "Content";
graphics.IsFullScreen = true;
}

Drawing 3D Wireframe model in XNA

in LoadContent(), set property RenderState.FillMode = FillMode.WireFrame;

protected override void LoadContent()
{
device = graphics.GraphicsDevice;
device.RenderState.FillMode = FillMode.WireFrame;
myModel = Content.Load("tank");
}