The initial clipping path gives you the size of the imagable area. Use ``clippath pathbbox'' to get these coordinates. If you must know the size of the device's imageable area, use the sequence ``gsave initclip clippath pathbbox grestore'', but this will prevent an enclosing application from using the clippath to achieve some special effects (such as multiple pages per page).
PPD files (see section 2 of the FAQ, printers) contain information on what paper sizes, as well as the Imagable Area for each, specific to each printer. A Postscript code fragment (called ``?ImageableArea'') is described in a PPD file, which determines the current Imageable Area for that printer.