To run ads in mobile devices, you should consider two main characteristics:
Mobile Ad recommended standard size:
Smartphones:
The majority of the devices in the market have a resolution between 300 px and 320 px wide, and between 50 px and 70 px high. Therefore, the recommended size for ads to cover the majority of equipment is 320x50 px.
You should bear in mind that this type of ads must be targeted by screen resolution: Maximum width of 320 px.
This format could be uploaded in the same space where you usually upload 728x90 or 300x250 ads. However, it should be targeted by screen resolution in order to display this version only when users enter the site using a 320 px wide device or lower.