If you set "width" and "height" in "New Document" dialog to 5.5 and 8.5 (inches?), it will definitely be that size. To test it, export your document to PDF, open it with Adobe Reader, go to File > Properties and see what dimension are displayed there.
As Nermander said, there is an option to adjust display size (in case you're measuring with a ruler): Preferences > Display. However, if you're measuring document size with a ruler, pay attention to zoom. But still, this is most unreliable method and I don't see any reason to use it. Scribus's rulers are, on the other hand, reliable, so, if they say 5.5 and 8.5, the doc is indeed 5.5 and 8.5.