I've tried to find more about this on both AdSense and AdWords and haven't found it.
I don't know when this appeared, but there is a link to "Creative sizes (Beta)" now.
Click on it and you get a list of ad sizes. The first on the list is "Dynamic".
What does the Dynamic listing represent?
At first I thought this is where we see which ad sizes appear in the new "Custom Ad sizes" ad blocks? But I haven't set up any of those.
Then I wonder if this also applies to using a 336x280 and getting the smaller 300x250 ad size served in that spot.
But I see ad sizes listed that I'm not using anywhere, like 250X250.
So what does this new feature/tool tell us, and what do we do with the new knowledge?