These aren't available from the AdSense add-a-gadget or ads-between-posts options in Blogger - but once you've been fully approved for AdSense, it's easy enough to add them to your blog by getting the code from AdSense, and installing it to Blogger the same way you install any other 3rd party code.
Personally, I like the 300x600 - it looks much more natural in several of my blogs, because it's more like the other things in the sidebar. Many of the ads it is showing at the moment are text-ads, because advertisers are still developing image-ads in the new size. But even the text ads look better, especially in sites where I am trying to blend ads with other content. (Believe it or not, I have one niche where many of the ads work the way Google originally thought of them, providing additional information that is genuinely useful to my readers.)
But you can have too much of a good thing.
AdSense have always had a rule that each screen should have no more than
- 3 ad-units and
- 3 link-units.
Now they have announced that from 10 January 2013, the rule is that each screen can have no more than
- 1 300x600 ad unit
- 3 ad-units
- 3 link-units.
This isn't a big issue for me, but obviously there are some publishers who have taken the p*** and devoted "too much" of their screen-real-estate to ads.
Notice that there are no changes to the rules about how many ads you can show from other advertisers and affiliate programmes.
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