Viewability 100% is calculated as Viewable-CPM and you will only pay for banners that are defined according to the IAB standard:
minimum 50% of the banners pixels are shown for at least 1 second.
Overall specifications apply and all measurements are in pixels (width x height)
File types: JPEG, JPG, GIF, PNG, 3rd party tags and HTML5*
Max weight: 120 KB per banner
*If HTML5 is submitted, the Xandr clicktag must be coded in.
Guide is here: Xandr HTML5 Clicktag Guide
When using HTML5 or 3rd party tags, 1MB can be politeloaded and additionally 4MB can be loaded on click.
Formats
930×180
160×600
320×160 / 320×320