Bad idea, I believe. The whole point in separating content from layout was to enable your web page to be displayed on any sort of browser.
Putting in artificial limitations such as a minimum screen size will limit your market.
Having said that, I believe every desktop should be able to display 1024x768. But what about the browsers running on iPhones or other screen-challenged devices, or even those that don't use their entire desktop for the browser?
In answer to your specific question, no, I don't believe there is any standard for a minimum or common display area in browsers.
