Which is the most targeted AdSense size? 338x280, 300x250 or something else? For me currently it's 250x250 looking at the reports
How do you mean? Do you mean a certain size will attract more targetted ads? If that's the case, then No. It makes no difference.
I mean when a advertiser wants to run CPM ads on your site which size is the most chosen one. With the new placement based targeting there might be some statistics...
I have always found the larger rectangle and skyscraper sizes to be best along with the leaderboard - it seems the larger the ad the higher the CTR...
728x90 has always been my best-performing! Also, the large square comes in a close second (can't remember it's size at the moment, though! ).