There are quite a number of sizes to choose from when creating an AdSense ad unit, and choosing the right one can make a big difference in your AdSense earning. Some AdSense unit size gets more click than others, and some advertisers even bid higher price for best performing ad sizes; giving you higher cost-per-click for your AdSense unit.
Many successful AdSense publishers has been reporting that their best performing AdSense ad units are the 336×280, 300×250, 160×560, and the 728×90. The common thing about these ad units are they are the biggest of their kind. Big ad units are more visible that their smaller counterparts, and they can also display more relevant ads at any one time.
When choosing the size for your AdSense ad unit, you'll notice that some of the available sizes are in the “Recommended” section of the list. The recommended sizes were indeed inline with AdSense publisher's report on their best performing AdSense unit size, and it's not just a coincident.
The following is Google's recommended sizes for text / text and image ad units,
and the following is for image ad units
Even though that some ad unit size generally perform better than the other, it's not all that counts. The best AdSense unit size is actually the one that can blend in well into your website's design.
Poorly designed websites (or too bloated with AdSense ads) would just drive away visitors, and would cause low rate of return visitors, and low inbound link / shares (thus low search engine ranking).