XP desktop > visible display size

How do I change the desktop size in XP? My XP desktop is wider than the visible screen on my new iiyama LCD monitor. It is clearly intended behaviour as the screen scrolls when the mouse pointer approaches either edge. I have set the horizontal size correctly. I have set the graphics card output to the native panel resolution and the frequencies are in spec. The drivers are fine. There are several similar questions on the web – but no answers! None of my desktop experts can help – and neither can microsoft. Yet another undocumented feature of XP.
All help appreciated.