What happens to Word documents with custom page sizes when they are converted?

PDFCreator is initially configured to recognize only the paper formats available at the time of installation. If a custom format is defined in Word, PDFCreator does not recognize it, meaning that Word will use the default page size. 

You can create new paper formats under: Print Management-> Printserver -> Forms.

"Print Management" is a Windows utility, where you can create name and dimensions of the form.