How commercial sites design their Web page content so that it will be functional on most computer monitors.

Basic Rule: It's not good if your page content is too wide for the computer monitor. That causes the surfer to have to scroll sideways to see the rest of the page. But in general, it is acceptible if your content is such that the user has to scroll down to see the rest of it.
Most sites don't favor page layouts that cover 100% of the width of the browser window because that leaves the look of the page dependent upon the surfer's whims. Moreover, resizing the browser window can cause the columnar layout to change unfavorably. Thus, most sites (who know what they are doing) choose to design a page with a tabular layout of a fixed width in pixels. But then, of course, the resolution of the computer monitor determines how wide the page is on the screen. The following stastics on monitor resolutions from thecounter.com were compiled during Febuary of 2004 based on millions of visitors to their site. (Presumably, they get the monitor resolutions of visitors from the JavaScript screen object.)

Monitor Resolution (WxH)

 Percent of Monitors

   640 X 480

1%

   800 X 600

37%

   1024 X 768

49%

   Other Sizes
(most over 800 pixels wide)

12%

Accordingly, most commercial web sites make their page content around 700-750 pixels wide so that it fits comfortably on an 800 pixel wide screen or wider. The page will then likely not spill off the screen horizontally on anyone's monitor.

Here are some examples of the page widths used by some major Web sites as of early 2004.

Yahoo.com: (740 pixels wide)
AOL.com : (705 pixels wide on an 800 wide monitor. - It looks like they have a script that sniffs out the screen width and delivers content whose width is based on that.)
msn.com : (779 pixels wide)
Microsoft.com : (Their headings are set at 100% of the window and the main content fixed at about 725 pixels)
Yahoo GeoCities : (750 pixels wide)
AltaVista.com : (750 pixels wide)
ebay.com : (760 pixels wide)
amazon.com: (this was actually 100% of the window -- a variable width page surprisingly)

It is interesting to see how page design has changed over time. Just a few years ago, most major commercial Web sites which used fixed width layouts designed pages that were around 600 pixels wide. Indeed, you can see why by looking at the stats I found sometime around the year 2000. Sites didn't want their pages spilling horizontally off of low resolution monitors, which then were somewhat more common.

Monitor Resolution (WxH)

Percent of Monitors

   640 X 480

13%

   800 X 600

54%

   1024 X 768

26%

   Other Sizes

 7%

It is interesting to look at the page layout widths used back then by major sites.

Yahoo,com: (640 pixels wide)
AOL.com : (585 pixels wide)
msn,com : (608 pixels wide)
Microsoft.com (Their headings are ste at 100% and the main content is 618)
Yahoo GeoCities : (675 pixels wide)
Lycos.com : (600 pixels wide)
AltaVista.com : (100% of screen)
ebay.com : (600 pixels wide)
amazon.com: (100% of screen)