As a rule of thumb, wider ad sizes tend to outperform their taller counterparts, due to their reader-friendly format. Readers absorb information in "thought units," several words at a time. Wider sizes let them comfortably read more text at a glance without having to skip a line and return to the left margin every few words, as they'd have to with a narrower ad.
If positioned well, wide ad sizes can increase your earnings significantly. The sizes we've found to be the most effective are the 336x280 Large Rectangle, the 300x250 Medium Rectangle, the 728x90 Leaderboard, and the 160x600 Wide Skyscraper. Keep in mind that while these ad sizes typically perform well, you should use the size that best complements your pages. For more information about these ad sizes, see our guide to ad sizes. It's better to add one recommended ad unit than two smaller ad units next to each other.
Tip for making the most of a little ad space: Try one of our streamlined link units, shown below. These sizes are versatile enough to fit in locations normal ads don't.