Google has unveiled the so-called "half-page" 300×600 unit for AdSense, claiming that one of the top requests from publishers in the last year has been for larger ad sizes in AdSense accounts.
To put it in perspective, see the above graphic. The half-page is that tawny-colored box in the center, offering as close to a full-screen takeover as a display ad can.
Google reports that in 2011, impressions via 300 x 600 grew by 37% among publishers, particularly among news, sports, and entertainment publishers. Still, they represented just 1% of impressions. The 300 x 100 format grew 70% in impressions, representing 2% of impressions.
The appeal of these premium ad sizes, says Google, is a more visually impactful size that is favored by brand advertisers, versus the functionality to direct response advertisers for which small ads are good enough.