See how many identifiers 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.
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 "Total size" column displays how many identifiers are in the segment and is updated multiple times per day.
For third-party segments, the "Total size" column shows how many identifiers 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 identifiers are stale or based in another country.