The functions shortside and sidelengths are generic.
  The functions documented here are the methods for the class "owin".
sidelengths.owin computes the 
  side-lengths of the enclosing rectangle of the window x.
For safety, both functions give a warning if the window is not a rectangle.
  To suppress the warning, first convert the window to a rectangle
  using as.rectangle.
shortside.owin computes the minimum of the two side-lengths.