• Optimize for 1004×598, which is currently the most widely used browser window size. Of course, the general guideline is to optimize for your target audience’s most common resolution, so the size will change in the future. It might even be a different size now, if, say, you’re designing an intranet for a company that gives all employees big monitors.
• Do not design solely for a specific monitor size because screen sizes vary among users. Browser window size variability is even greater, since users don’t always maximize their browsers (especially if they have large screens).
• Use a liquid layout that stretches to the current user’s browser window size (that is, avoid frozen layouts that are always the same size).

My modified version of Jakob Nielsen’s Alertbox “Screen resolution and page layout“, July 31, 2006. My modifications: browser window size instead of screen size.

• Easier to move from one line to the next
• It is much faster to read in general
• The brain use less mental effort (which increases understanding and learnability)

Sizes to design for

* {
margin: 0;
padding: 0;
}