AllowUserResizing controls whether the user can resize the game window by dragging the edges or corners. This value is false by default.
ClientSizeChanged is an event which is raised whenever the window is resized. Unfortunately this event is only raised when the window is resized by dragging the corners/edges, or when the window is maximized. Once the window is maximized, clicking the maximize button to restore the window will not raise this event. Therefore, it is recommended that Windows Desktop games the CornerGrabbingResize event. For more information on this event, see the this page.
The GameWindow class represents the window that the game is rendering at. This is typically accessed through the XNA Game's Window property.
The resolution of the game window can depend on the presence of splash screen files. For more information, see this discussion: http://community.monogame.net/t/not-correct-bounds-size-on-iphone5-always-480x320/591