There isn't an agreed standard, I'm not sure there is even a widely-agreed ideal size. I've seen plenty of referrances to 960px as a good width; Sites like Youtube and Facebook have used that as the width of the content, as well as there being templates systems based on that width. I don't know why, though. It might be the result of a statistics.
Here's a little something from Google: Screen Sizing
Regarding height; It's been suggested the top 600px or so will be the area of first impression, even on larger resolutions, but generally speaking there will be no standard there. Scrolling is already standard enough.