Google’s second recommended ad unit size for AdSense is the 336 x 280 – Large Rectangle. This ad unit size is not listed in the most popular sizes on the Yahoo! Ad Network, but is on the listed in the Microsoft Advertising PubCenter for ad publishers for small businesses, but not in the Standard Ad Creative Acceptance Policy for Marketers & Agencies. On the IAB (Interactive Advertising Bureau) Ad Unit Guidelines, this size is listed in the Delisted Standard Ad Units as of February 28, 2011.
While this size is available in AdSense for both text and image/rich media ads, it seems that this size is used mainly for small publishers and due to it’s size, is commonly used to take up a large area of the content. Google offer examples of the 336×380 Large Rectangle and they are all embedded in or below the content.