code: func (f *Fpdf) PageSize(pageNum int) (wd, ht float64, unitStr string) {
  	sz, ok := f.pageSizes[pageNum]
  	if ok {
  		sz.Wd, sz.Ht = sz.Wd/f.k, sz.Ht/f.k
  	} else {
  		sz = f.defPageSize // user units
  	}
  	return sz.Wd, sz.Ht, f.unitStr
  }
docs: // PageSize returns the width and height of the specified page in the units
  // established in New(). These return values are followed by the unit of
  // measure itself. If pageNum is zero or otherwise out of bounds, it returns
  // the default page size, that is, the size of the page that would be added by
  // AddPage().