See how many cookies are in a segment
This feature might not be enabled in your DFP environment. For more details, talk to your manager or a DoubleClick representative.
You can see how many cookies are in a particular segment on the "Audience segments" screen.
To go to the "Audience segments" screen, click the Inventory tab and click Audience segments in the left navigation pane.
For first-party segments, the "Size" column displays how many cookies are in the segment and is updated multiple times per day.
For third-party segments, the "Size" column shows how many cookies are provided in the segment but doesn't necessarily reflect the total amount of unique impressions you'll get from the segment. This could be because some cookies are stale or based in another country.