[PDF file doesn't show document's actual size]

I've designed a business card and I need to export it as a PDF document for printing. My printing lab asked me to add the card's actual dimensions (85mm x 55mm) into the document, but I couldn't find the way to do so. 

 

The only shown information related to document size is resolution (240 x 156), which doesn't help me in this particular case.

 

How do I add 85mm x 55mm into the document, so that when printing, the printer automatically knows how large or small the print should be?

 

 

Thank you!