Xara Web Designer does not produce pages that stretch to fill monitors, AKA: Fluid or liquid layouts
Xara Web Designer creates WYSIWYG fixed width sites with absolute positioned page elements.
Fliud/liquid layouts are not WYSIWYG.

Essentially, when you use a liquid layout, you have less control over the appearance of your page. It will appear as wide or as narrow as your visitor's browser. As a result, when the browser window gets too wide, everything will appear stretched out. When the browser window gets too narrow, your text and graphics in the various columns will suddenly be horribly misaligned.

With a fixed layout, the elements in the page will retain their proportion regardless of the visitor's screen resolution. Of course if the resolution drops below the fixed width you assigned, the visitor would have to scroll horizontally. But at least your page still appears as you planned.