Per this media post article, ad size is not always a good gauge for success, at least as it pertains to rich media.
“Eyeblaster data suggests that physical size of a banner has less to do with performance than creative execution and rich media,” said Gal Trifon, CEO and co-founder at Eyeblaster. “This corroborates other published studies that reveal that bigger banners do not equate to better engagement.”
With all the budget cutting in all areas of online advertising, this is interesting in that creative is still measured as being important. Seems obvious to me as a consumer, but based on all of the media focus in the industry, it would seem to put creative entities in demand.