The adsense have released a new ad format, smaller than its current 300x300. Great addition given how difficult it is to design well balanced pages.

The 200x200 is unlikely to be a hit in terms of clickthrough, Google has published its highest performing ad sizes:

• 336x280 large rectangle
• 300x250inline rectangle
• 160x600 wide skyscraper

The graph shows the best performing locations for adsense; dark orange is the strongest performance location, while to light yellow is the weakest performance. Naturally, ad placements above the fold tend to perform better than those below the fold. Ads placed near rich content and navigational aids usually do well because users are focused on those areas of a page.

But nothing beats trial and error with web analytics of user behaviour. [Via Google's Inside AdSense: New channels page, new ad format]