(glib:define-gboxed-opaque paper-size "GtkPaperSize"
  :type-initializer "gtk_paper_size_get_type"
  :alloc (cffi:foreign-funcall "gtk_paper_size_new"
                               :pointer (cffi:null-pointer)
                               :pointer))  

The gtk:paper-size structure handles paper sizes. It uses the standard called "PWG 5101.1-2002 PWG: Standard for Media Standardized Names" to name the paper sizes and to get the data for the page sizes. In addition to standard paper sizes, the gtk:paper-size structure allows to construct custom paper sizes with arbitrary dimensions.

The gtk:paper-size instance stores not only the dimensions (width and height) of a paper size and its name, it also provides default print margins.