There is nothing you can do from your end - it is up to the web site designer.
Some use relative sizing, that allows the site to fill the browser window. Others use fixed widths - these normally state 'best viewed at 800 x 600', or similar.
It is very difficult to preserve the exact intended layout and look of a site if your site has to reach certain design objectives at both 640x480 and 1600x1280.
Google, for example, has a very simple, clean text-based layout, so it can use a relative or dynamic sizing method. MSN, by contrast, is graphics-intensive and obviously wants to keep the alignment and relative position of the page components consistent.
C'est la vie!