As we know Google AdSense surprised us by number of changes they have introduced in last one year, and now it comes with an another change is that every publisher can set custom ad size and place into their blog. this is really good opportunity for the publishers to improve their earnings and better design of content for the readers. This feature was available for the premium publishers only and now Google AdSense given this feature for the normal publisher too. but still there is a limitation of ad units that we can display on our blog.
Related : Google AdSense Pop-up Window Ads
How to Get Started with Custom AdSense Ad Size Unit?
1) Go to Google AdSense and as usual, Create an ad unit that option available into the “my Ads” tab.
2) Choose“Custom Size” from the Ad size drop-down and set width and height which you have desired for your ad unit.
3) Also there is many customization options are available that you can use according to your needs and then finally click on “Save and Get code” button to get ad code and place it in your blog/website where you want to the ad to appear.
4) Now, wait for the few minute and then your ad will be displayed with custom ad size.
for the more information you can check Google AdSense official blog : http://adsense.blogspot.in/2013/12/the-next-evolution-of-responsive-ads.html