How do you save the size in word, before converting to a PDF?

I created a bookcover and saved it in word. The publishing company sent over a PDF template for me to paste the cover within their margins. When I checked the size of that PDF it says 13.9 x 11. I exported that PDF to word and added the bookcover in the template. I embedded the fonts %26amp; saved all changes. Next I went to print, selected adobe printer, and clicked properties. Then I created a custom ';default setting'; adding the size 13.9 x 11 (which was already preset to letter size 8.5 x 11). Then when I tried to create (add) the custom size under the last box called ';Adobe PDF change size';, the custom size won't pop up in the drop down menu. Therefore, when I convert this document to a PDF, the size defaults back to letter size 8.5 x 11. Please, please, please help me!!!! Thanks!!!How do you save the size in word, before converting to a PDF?
Hire a designer. You should not be trying this in Word.