Larger ad sizes tend to generate greater response

"Larger ad sizes tend to generate greater response. Size does matter apparently. As we have seen in years past, bigger ads perform better. We observed this with the latest set of metrics in which the half-page ad unit (300x600) which had the highest CTR, interaction rate (for both in-page and expandable formats) and expansion rates of all the available creative sizes we tracked for the benchmarks.
Although the U.S. had one of the lowest expansion rates, this market showed the highest expansion time. So Americans seem to expand ads less but when they do spend more time engaging with the ads themselves.
In terms of industry verticals, auto advertisers performed the best in terms of CTR (0.13% for flash ads) but conversely had the lowest interaction (1.9%) and expansion rates (0.2%). For interaction rate, telecom (9.4%) and B2B (9.2%) advertisers were the highest while B2B stood out in terms of expansion rate (7.5%) benchmarks."

Source:  Global data from DoubleClick, revealed in a blog post, 16th August 2011

Methodology:  "A couple of notes about this latest set of online advertising benchmarks. The data for these benchmarks are derived from a robust data set across DoubleClick for Advertisers, based on rigorous methodology with input from the Advertising Research Foundation. The charts cover global benchmark figures for the entirety of 2010 by ad format, ad size and industry vertical. The benchmarks are normalized across hundreds of advertisers, thousands of campaigns, and tens of billions of ad impressions."