Best performing ad units for AdSense are 160×600, 336×280 and 300×250. 2 best performing square ads are 300×250 pixels and 336×280 pixels. For wide sky 160×600 pixels is best.
Where do you place Google ads on your pages? Here is the Google Adsense “heat map” that illustrates the ideal placing of ads, it suggests to place ads directly below the end of the editorial content.