Cairo cannot set the page size in the pdf for proper output. A way of specifying the paper size in points to the pdf surface is needed. This would allow cairo to emit a /MediaBox or somesuch for non-standard paper sizes.
I'm not sure I understand what you're asking for. You set the paper size in PDF points when you create the PDF surface. Do you want a way to set a /CropBox different from the page size?
Closing due to inactivity.
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