The bottom line is, there is no true standard web page size. Nearly all web browsers are set to display a resolution of 800 x 600 pixels or higher and a most users have 1024×768 or higher resolutions. Most mobile browsers are set to view from 640px by default.

The graph above shows how no visitors to this website have their settings less than 1000 pixels. Whilst there are some users who are still at 800 x 600, we encourage web designers to move towards the minimum width of 1024.

UPDATE:

As this page is getting some hits I thought I would clarify a few things. First there really is no “standard web page size” and if you seriously looking into web design, then the less standard you can make your website the better. Second there have been some important changes to the way web pages are displayed that make the idea of a standard web page size almost obsolete.

Responsive Web Design

A combination of good CSS3 and code, responsive web design means that parts of your site will adjust depending on the screen size.  Smashing Magazine has a great break down on what this is. Art Equals Work is a great example of this, if you go to their website, resize your browser and see.