What is a 320×100?
This ad size is known as a Large Mobile Banner and is the big brother of the 320×50 and 300×50 ad units. A 320×100 ad unit is most commonly used as an ad on smartphones and is the largest of the mobile-specific banner ads.
What it looks like
The large mobile banner dimensions are 320 x 100 pixels.
Note: The live ad may not always load for technical reasons, and instead a backup image will show. Because AdSense runs this as a multi-sized ad slot, a 320×50 will occasionally appear floating in the space.
320×100 Advice for Site Owners
A 320×100 is listed as a top-performing ad unit by Google AdSense, so is probably worth trying for websites with a lot of mobile traffic. This fact alone means it should certainly be implemented instead of the smaller 320×50 if possible.
It is unlikely to be an especially strong revenue driver as it is not a very large ad. This is because larger ads perform better than small ones. It is potentially a good way to monetise mobile ad impressions, however.
Note: These ads are often run as “Anchor units”. This means they float on top of the webpage at the bottom of the screen. If you implement this sort of ad, make sure there is an easy way to close the anchor unit. If you don’t then you risk overly annoying your users.
320×100 Advice for Ad Buyers
320x100s are small ad sizes, but the largest available mobile-specific banner. This makes it likely more effective than other mobile banners, but that effectiveness likely comes at a price. If you are able to buy this ad size at a price that ensures you a positive ROI, then we highly recommend it. If not, then perhaps just stick to 300x250s, which can run both on mobile and desktop devices.
To get the most out of a 320×100 you have to make the best use of the small space available to you. This means having a very concise message and call to action, which can fit in the space.