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 336 x 280 large rectangle, the 300 x 250 medium rectangle, the 728 x 90 leaderboard, the 300 x 600 half page and on mobile the 320 x 100 large mobile banner. Bear 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. It's better to add one recommended ad unit than two smaller ad units next to each other.
Source : https://support.google.com/adsense/answer/17955