As David says, it all depends on your viewing audience.
Have a look at the stats for the site (if you have any at this point). What type of people are going to be viewing the site. If it's a more technical audience, then they probably have pretty wide monitors, but it's a 'mom & pop' type site, then they may still be using small monitors.
These days I've gone from 780 width (for 800px wide window size) to 980px width to cater for at least 1024 window size. As for height, don't declare a height at all. This will allow the content to expand vertically as needed regardless of window size.
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