How do I set the window / screen size in xna?

As of XNA 4.0 this property is now found on the GraphicsDeviceManager.
Ie. this code would go in your Game’s constructor.

graphics = new GraphicsDeviceManager(this);
graphics.IsFullScreen = false;
graphics.PreferredBackBufferHeight = 340;
graphics.PreferredBackBufferWidth = 480;

// if changing GraphicsDeviceManager properties outside 
// your game constructor also call:
// graphics.ApplyChanges();