Although this is an old thread, I ran into exactly the same issue with Word 2010 and Distiller 10. All settings in Distiller, properties, Word page properties, print options pointed to Letter size paper, but when printed to PDF the page was coming out as A4. Other applications seemed to print correct Letter size.
The solution that worked was to change the change Word properties. In Word 2010 the setting can be found under:
File -> Options. Pick the Advanced item on the left pane, scroll down to Print section and uncheck the "Scale content for A4 or 8.5 x 11 paper sizes" option.
Hope it helps.
That was exactly the information I needed for a colleague; using FrameMaker or LibreOffice, I hadn't hit this particular problem. Hapless colleague, obliged to work with Word 2010, was going hairless ... until he asked me, and a quick search turned up this post. Result: one happy (or at least happier) colleague. Niels Grundtvig Nielsen