Viewability 100% is calculated as Viewable-CPM, and you will therefore only pay for ads that are visible, defined according to the IAB’s industry standard:
A minimum of 50% of the banner’s pixels are displayed for 1 second or more.

File types
.JPG, .GIF, .PNG, 3. parts tags or HTML*

Formats
930×180
160×600
320×160/320×320
(All measurements are in pixels (width x height))

Weight
Max 120 KB per banner
If HTML or tags are used, 1 MB can be polite-loaded and 3 MB can be downloaded if the user clicks on the banner

*Clicktag
When using HTML or tags, a Google clicktag must be coded in the banner.