Pressing key [F] "toggles" between full screen and windowed mode.
[O] is pan-scan and [M] movie mode ([H] to hide/show control panel)
Check (i.a.) setting of "[ ] Always start in full screen mode" under
(rightclick >) Options > Preferences > Video
Further see this post
http://forum.bsplayer.org/viewtopic.php?p=15316#15316