If you try blenderplayer --help
you will find the following options
-w: display in a window
--Optional parameters--
w = window width
h = window height
l = window left coordinate
t = window top coordinate
-f: start game in fullscreen mode
--Optional parameters--
fw = fullscreen mode pixel width (use 0 to detect automatically)
fh = fullscreen mode pixel height (use 0 to detect automatically)
fb = fullscreen mode bits per pixel (default unless set in the blend file: 32)
ff = fullscreen mode frequency (default unless set in the blend file: 60)
These options can be added as Popen(['blenderplayer', '-f'])
You can also switch fullscreen on/off from within the game by using bge.render.setFullScreen(enable)
within your game scripts.