Does size matter? Understanding ad sizes

Selecting the right ad sizes can greatly impact the effectiveness of your display campaign.

As a general rule of thumb, the more sizes in your ad set, the greater the reach of your campaign will be. The reason for this is simple: The ad slots (or ad inventory) on the webpages your ads will be published on may vary. However, creating too many sizes may have a negative impact on your ROI.


Keeping an eye on which ad sizes perform best allows you to reassign budget from poor performing sizes to better-performing ones. The performance of ad sizes is caused by several factors, which we will discuss in this post.


Working with an e-commerce retailer in consumer electronics, we found that the smallest and cheapest size (320x50) outperformed all other sizes in the set. Surprisingly, the largest and most expensive size (300x600) performed the worst. By tossing out the 300x600 and spending more on the 320x50, the campaign’s ROAS improved greatly.


The most popular ad sizes

According to Match2One, 89% of all ad impressions are generated by these 4 sizes:

          • 300×250 Medium Rectangle (40%)
  • 728×90 Leaderboard (25%)
  • 320×50 Mobile Leaderboard (12%)
  • 160×600 Wide Skyscraper (12%)


Google offers a similar list of top-performing ad sizes:

  • 300x250 Medium rectangle
  • 336x280 Large rectangle
  • 728x90 Leaderboard  
  • 300x600 Half page
  • 320x100 Large mobile banner


Including just these ad sizes in your set limits the reach of your campaign. For optimal reach, you should include at least a handful more sizes. A study by AppNexus has revealed a correlation between the number of ad sizes and the performance of your campaign as shown in the visual below.


Screenshot 2019-08-05 at 09.29.17

Which sizes you will need depends on your industry, geography and target audience. Once you’ve figured out which ad sizes work for you, you can start tossing out the ones that aren’t performing well enough.


Niche markets, odd sizes

Niche websites often attract hard to reach audiences. They are also contextually interesting because your offering is likely to be highly relevant here. Often these websites have odd-shaped ad slots that are easy to miss when you haven’t created enough sizes in your ad set. 


It pays to do a bit of research on the ad slots on these websites. Having the appropriate ad sizes in your set will assure that your ads will be served to your desired target audience. You may even be able to find some ad slots for which there isn’t a lot of competition, which means they will be more cost-effective.



If you’re advertising in Europe you should take into account that some countries in the Nordics and Eastern Europe have specific ad sizes. Google has listed some of them. If you are advertising in one of these countries, be sure to include the right ad sizes in your set.



An increasing number of impressions, clicks, and conversions take place through mobile devices. For some markets, a display ad is more likely to be viewed on a mobile device, than on desktop. The best way for you to find out if this holds true for your industry is to include two or more mobile ad sizes at the start of your campaign. After you start seeing some results coming in, you can start removing poor performing ad sizes. 


If mobile ad sizes prove to be your best performing ads, you may want to consider starting with the design of your smaller sizes first before creating the larger sizes. It also can’t hurt to do some more research on mobile ad sizes and find out how you would like your mobile ads to be placed. Google suggests that your 300x250 ad is best placed just above the fold, but not all the way as shown in the image below.





Diminishing returns

Simple opportunism may tell you to create as many sizes as possible in order to obtain maximum reach. However, creating all these ad sizes will cost you time and resources. 


Moreover, Google indicates that many ad sizes may perform sub-optimally because of limited ad space or their small size. If you can reach 89% saturation of ad impressions with only four sizes, creating ten to fifteen ad sizes should be more than enough. After that, all the time and budget you put into creating additional ad sizes will cause your returns to diminish.



As a rule of thumb, wider ad sizes tend to outperform their taller counterparts, due to their reader-friendly format. Find this and other interesting facts in Google’s Guide to ad sizes.


Get insights 

The best way to go about picking the right ad sizes for your campaign is to build ten to fifteen different ones and work your way back from there. This will provide insight into where your budget is spent inefficiently and where you could allocate more. Just running four to five ad sizes is not enough and more than fifteen will probably be too much. Taking into account some factors like relevant niche websites, geography and mobile will help you make sensible choices.

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