I've been using this freeware app (
http://www.brianapps.net/sizer/) to resize my windows to specific sizes so I can get an idea how it will look at a certain resolution.
For the dr800/iliad, I've created profiles for portrait 768x1024 and landscape 1024x768. Note that you may want to add to the numbers to compensate for PC-specifc things like toolbars, window bar, etc. And you have to consider reader-specific features such as being able to fit-to-text which ignores the useless borders in PDFs, status bar at top, toolbar at bottom, etc.