In my experience with using AdSense, it seems the best AdSense sizes are the skyscrapers, 120x600 and 160x600.

I haven't yet figured out which of the two is better.

Keep in mind however, that the best sizes will vary depending on the design of your website, and the placement of the AdSense unit. In reality, other sizes such as the 728x90 or the 300x250 may outperform the skyscrapers based on site design and ad placement.

It's also well accepted that AdSense units placed within the content, particularly inside the body of an article, outperforms skyscrapers placed on a side column. In that sense, the square-shaped sizes such as the 300x250 will outperform skyscrapers when used in this way.

If you publish a blog, you'll know that placing a 300x250 unit will sometimes interfere with images appearing inside the article. For this reason, bloggers tend to shy away from using in-content placements.

Take a look at the permanent URL for this article (click here) and notice that the upper-right of the article has a 200x200 AdSense unit, and that I never place an image at the top of the article. What images I may use, always appear well into the middle of the article so as not to interfere with the AdSense unit.

Still, I'd give the skyscrapers the title of the "best AdSense sizes" because they do perform well in side columns, and because they're in side-columns, you never have to worry about them interfering with content. It provides for greater artistic flexibility that way.

Keep in mind another general rule of thumb for identifying the best AdSense sizes are those sizes that display 4 to 5 ads per size.

Labels: AdSense Sizes