cairo:pdf-surface-set-size (surface width height)

surface -- a cairo:surface-t instance
width -- a number coerced to a double float for the new surface width, in points (1 point == 1/72.0 inch)
height -- a number coerced to a double float for the new surface height, in points (1 point == 1/72.0 inch)

Changes the size of a PDF surface for the current (and subsequent) pages. This function should only be called before any drawing operations have been performed on the current page. The simplest way to do this is to call this function immediately after creating the surface or immediately after completing a page with either the cairo:show-page or cairo:copy-page functions.