I don't know the average size but I do know that you should consider how wide you want your site to be. I believe the last time I checked my Google Analytics stats, only 5-10% of my visitors are viewing my site with a resolution below 1000 pixels wide. So I no longer worry about the folks with the 800x600 browser resolutions as much anymore. I usually try to shoot between 900 and 1000 for my site's width.
So I think it really depends on how wide you want your site to be. There really is no right or wrong unless you choose to make it 1700 pixels wide when most people don't have monitors with such a large resolution.
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