Placement of watermark depending on document size?

I'm pretty new to PostScript and need help woth the following. I need to watermark some PDFs and have made the necessary changes to the prologue.ps-file. But the thing is that I need the placement, and maybe even the font-size etc., to adjust to the size of the document that is being printed through the distiller. Is there a way to write something like

if( PDF.Size=A4){
{ gsave
/Helvetica_Bold 220 selectfont
.85 setgray 130 70 moveto 50 rotate (DRAFT) show
grestore
} bind
} else {
{ gsave
/Helvetica_Bold 300 selectfont
.85 setgray 180 100 moveto 50 rotate (DRAFT) show
grestore
} bind
}

I know THIS wont work, but I hope you understand what I want to achieve!! =) Thanx for any ideas!!

Best regards,
J