  plgspa(xmin, xmax, ymin, ymax)
  ******************************

  plgspa

  Gets the size of the current subpage in millimeters measured from the
  bottom left hand corner of the output device page or screen. Can be used in
  conjunction with plsvpa for setting the size of a viewport in absolute
  coordinates (millimeters).

  xmin (PLFLT *, output): Pointer to variable with position of left hand
  edge of subpage in millimeters.

  xmax (PLFLT *, output): Pointer to variable with position of right hand
  edge of subpage in millimeters.

  ymin (PLFLT *, output): Pointer to variable with position of bottom edge
  of subpage in millimeters.

  ymax (PLFLT *, output): Pointer to variable with position of top edge of
  subpage in millimeters.

