Last month, Google introduced a new ad size for AdSense with the 300X600. The response to the new format seemed pretty good, but perhaps it was too good, because Google has made a change to its policy to ensure publishers only use one of these units per page, at most.
"Because this new format is larger than all our other ad units, we want to help ensure a balance between content and ads across publisher sites," says Google policy associate Charles Huang."With this in mind, going forward, publishers will be permitted to place a maximum of one 300x600 ad unit on each page. If you’ve placed more than one 300x600 ad unit per page on your site, we ask that you remove any additional ad units by Jan 10, 2013. We’ll also be sending email notifications shortly to affected publishers, asking them to make this update."
"Please note that the limit for AdSense for content ad units still remains at three per page, so it’ll be possible to place one 300x600 ad unit and up to two other sizes on a page," he adds. "As always, it’s important to consider both the user and advertiser experiences when placing any additional ad units on your pages."
You know that Google doesn't like pages that don't have enough content "above the fold," so it's no surprise that Google does't want publishers plastering these bad boys all over their pages.
Google's guidelines for ad placement can be found here.