WindowSize is an option for Notebook which specifies the size of window that should be used to display a notebook on the screen.
WindowSize->
w
,
h
gives the width and height of the window in printer's points.
Setting either width or height to Automatic causes the size of the window to be determined from the setting for WindowMargins and the size of your screen.
The setting for WindowSize changes whenever you resize a window interactively in the front end.
On most computer systems, the front end does not allow window sizes below a certain minimum value.
w
,
h
gives the width and height of the window in printer's points. 
