It depends on who you are listening to.
The consensus is that bigger is better. Many successful AdSense publishers has been reporting that their best performing AdSense ad units are the 336×280, 300×250, 160×600, 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.
According to google adsense support:
“The sizes we’ve found to be the most effective are the 336×280 large rectangle, the 300×250 medium rectangle, the 728×90 leaderboard, the 300×600 half page, and on mobile the 320×100 large mobile banner. ”