Back Buffer is the surface area on which XNA constructs the graphics which we want to display in the Draw method.
800*40 is the by default back buffer size.
Here we set the width and height of the Back Buffer:-
graphics.PreferredBackBufferWidth=100;
graphics.PreferredBackBufferHeight=200;
There are another property called PreferredBackBufferFormat, which is used to set the format of the back buffer.