As of Oxygen 3.1 RC1 not all registered (default and custom) image sizes are available in the Insert Dynamic Data > Featured Image and other places where featured image or other dynamic images' (like coming from a custom field) URL can be set. Bug report in Github.
In this members-only tutorial I show a workaround wherein we can define a custom function that returns the URL of the image (featured image in this case) in the specified size.
Once the function is in place, we can call it via data > PHP Function Return value.