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So you are looking for the best performing ad size. Ok here you go. The best performing adsense ad size is : Large Rectangle (336 x 280) .

Large Rectangle (336 x 280) gets the most clicks. It doesnt look like the standard ads people got used to. They perform best when you place your ad floating inside the content. I highly suggest that you kepp your adsense floating on the left side. People start reading from left (its the opposite in languages like arabic) so your ad will attract more attention. Here is the code that will float your ad on the left side and wrap the text around it. You can adjust the margin size as you like to keep your page neat. The numbers are given to give you some idea.

<div style="float: left; margin: 3px 3px 3px 3px;">

Here is the code that will float your ad on the right side and wrap the text around it.

<div style="float: right; margin: 3px 3px 3px 3px;">