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;
}