It sounds like you need a fluid CSS page design, so that your page flows to fill whatever size/shape screen it is displayed on. You can find lots of posts about fluid design over in the CSS forum: [webmasterworld.com...]
Most web sites have pages with variable page lengths to accomodate the content, so only the page width is an important factor when designing.
If you are using a fluid-width design, the content will flow and fill the horizontal space. This can be a problem on short pages viewed full-screen on wide screens - it is important to consider what will happen to floated elements etc. if the page is horizontally stretched.
If you are using a fixed-width design (usually at less than 800px) then it will only fill a very small part of the screen real estate when viewed on a high definition wide screen. This problem is mitigated by the fact that the vast majority of wide screen users will not have their browser window stretched full width.