Google released it’s largest ad unit ever in late 2012. It’s 300×600 and is called the Large Skyscraper. To test it out, I added it to one of my “project websites” and here is what I found:
1) If you set it up to display Text Ads Only, you get eight Text Ads and everything looks fine.
2) If you select Text and Image/ Rich Media ads, you most likely will see a lot of empty space to both sides of the Image/ Rich Media ads. This is most likely due to advertisers who just haven’t started using the new unit yet. Instead of not showing any advertisement at all, Google is simply inserting Image Ads from the 160 x 600 Wide Skyscraper unit, which gives 70 pixels of empty space on each side. You can see what I mean at www.sportscardsstores.com.
3) You can only have one 300 x 600 unit per page.
This is the largest Google Adsense unit ever created and is most likely is not going anywhere. I expect the empty space issue to gradually go away once more advertisers start using the new unit. It does take up a lot of space so if you decide to use it, I suggest putting it off to the side and not in your content area.