AdSense has introduced the next step in responsive ads which is customer ad sizes.
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