A post on he AdSense blog tells us
Today, we’re happy to announce the launch of 434,216 new ad unit sizes! Or in other words, we’ve just made it possible to create custom ad sizes to give you more flexibility and enable you to better design your content for your users.
Custom sized ads behave just like their standard size counterparts, showing a text or display ad depending on which is likely to have the best performance for the impression.
For text ads, we’ll determine the optimal number of text ads to display for a given impression, and show the ads the same way they appear inside standard sizes. Our system predicts the value of each competing ad for every impression, based on historic performance and comparison across a variety of scenarios. For unique ad unit sizes, our system will need some time before it can optimize the number of ads shown.
This is how those text ads might look
The same is happening for display
To ensure that ads continue to deliver a positive user experience, we’ve put a few restrictions in place around maximum and minimum pixels for ad sizes. Any custom ad size that falls outside of the restrictions won’t appear on a page. As always, all of your ads must adhere to the AdSense ad placement policies. Remember that users value your content, and that they appreciate a balance of content and ads, knowing that the ads support the content creator. Forcing too many ads into the users viewable screen is not likely to tempt them back to your site.
So many options. How are you going to respond?