Google announced today that it is now offering custom ad sizes for AdSense, or as the company says in a blog post, 434,216 new ad unit sizes.
"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," Google's Omar Gimenez Llach and Johan Land explain. "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."
"For display ads, our system will determine the appropriate size ad to fit into the space you select; the selected ad will not be larger than the space requested," they say.
To create a custom ad size, select "Custom ad size" from the Ad Size drop-down, and set the dimensions.
There are some restrictions for maximum and minimum pixels. Only a single dimension can be over 300, and no height or width can be over 1,200 pixels. Also, the minimum width is 120 pixels, and the minimum height is 50.
Just remember how Google views ads above the fold.