Typically, you want your content to appear on all monitors of a reasonable size without the need for horizontal scrolling. One way to do this is to make sure your content is not wider than a certain number of pixels (say, 760 ishs for 800x600 or 960ish monitors if you want to configure 1024x768 as the minimum monitor size).
Another option is liquid layouts, which extend to the size of the browser window, but are usually harder to code and often equally undesirable for very large monitors (do people really want the site to be stretched at 1900x1200 resolution?).
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