I figured out myself. For some reasons I can't understand if I set the page size as a style form resources, it works on printer and on the DocumentViewer, but it fails when it exports the xps. To fix this I just specified the size in each page:
<FixedPage Height="816" Width="1056" >
It is a little silly because I know in advance how big each page is, so having a type based style seems a good idea to me, but it did not work.