The placement policy for the new Adsense ad format 300x600 has been updated by Google, that webmasters / bloggers are "permitted to place a maximum of one 300x600 ad unit on each page", said by Google yesterday in it's official blog post Updates around usage of 300x600 ad format - Inside AdSense.
If you’ve placed more than one 300x600 ad units on a page on your website / blog, Google
asks that you remove any additional ad units by Jan 10, 2013.
Since the overall requirement for AdSense placement for content is limited to three ad unites per page, so if a 300x600 ad unit is already on a page, then you can use up to two other sizes of ad units on that page.
Google said doing this is to "ensure a balance between content and ads across publisher sites".
Popular ad formats supported by Adsense including: Leaderboard (728 x 90) , Banner (468 x 60) , Skyscraper (120x600) , Wide Skyscraper (160x600) , Small Square (200 x 200), Square (250 x 250), Medium Rectangle (300 x 250), Large Rectangle (336 x 280) and some other formats. You can find out the example of all above format on this Adsense Help page. Unfortunately, the 300x600 format does not have an example yet on that page.
300x600 format is one of the fastest growing ad sizes by impressions on the market, and is preferred by brand advertisers. It has been provided by advertisers and other ad networks for some time.