Web publishers have been bugging Google for quite some time to increase the size of it's largest ad unit. One of the top requests from website publishers in the last year has been for larger advertising sizes in AdSense accounts.
Google has decided to comply. The search giant has released a half page ad unit for Adsense that is as close to a full-page, take over as display can get.
Impressions via the 300 x 600 ad unit size grew by 37% among publishers in 2011. The growth was particularly clear in verticals such as news, sports, and entertainment. Ad units with more visually impactful ad sizes are favored by advertisers and publishers, as they are the fastest growing size ad unit by impressions.
Changes in CPM By publisher country within the ad exchange
SOURCE: Display Business Trends Publisher edition by Google and Doubleclick
https://static.googleusercontent.com/external_content/untrusted_dlcp/www.google.com/en/us/doubleclick/pdfs/display-business-trends-publisher-edition.pdf