The default window positions are stored in the first four entries of the appropriate line in the [windows] section of $mircini - e.g.
//echo -a $replace($gettok($readini($mircini,windows,wchannel),1-4,44),$chr(44),$chr(32))
gets the x y w h values (and the $replace formats them for use with /window, no less).
NB. Any channel that you give a saved position for will have its own entry in the [window] section (#chan_name instead of wchannel)
If I have the inclination later, I might eve write a "reset to default" menu option using this.