The big is the best! YES big is best. If you fellow this instruction from google? Some people ask this question why google need to reveal this secret to their publisher and sharing this money to them. Of course without sharing a good success tips, google might not reach the successful jobs like today. Sharing the BIG secret mean to win huge billion of $$$
For adsense publisher adsense recommend the BIG ads space like 300 x 250 the medium rectangle, 336 x 280 the large rectangle, 728 x 90 the leaderboad, and the skypecraper 160 x 600.
So can you be the one to make a testimony of these ads size too? I believe they are telling the publisher to make good money once they have more traffic to their site especially the organic traffic from search engines.
These traffic will triple more to their earning with these ads size from the recommend inside google adsense setup code.
Most of my blog and websites (a few), I only use these recommend size and it performs very well indeed. You can try one of those or all of them and see how it work.