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