(Default key is [A]; see also
http://forum.bsplayer.org/viewtopic.php?p=13005#13005 )
rightclick > Options > Preferences > Key definitions & WinLIRC
then select "Cycle aspect ratio"
and (re)define rc key which you prefer
(Place cursor in box below "Or press button on remote" ... (etc))
Note keys are (to be) defined for Windowed mode and Full screen mode seperately!