For all Website Audiences that you define in AdRoll, you will be able to see two different metrics: Estimated Audience Size and Unique IDs.
Navigate to Audiences > Website Audiences > Configuration.
It is important to understand the differences between Estimated Audience Size and Unique IDs
Your Estimated Audience Size is the number of visitors that are currently tracked in a Website Audience today indicating the total number of cookies available that can be targeted if you use this audience. It is calculated based on how long you keep a person in an audience by adjusting the Audience Duration.
Unique IDs are the number of unique people tracked during the date range selected in the date selector found at the top of the Audiences> Website Audiences page. For example, if you select Last 14 days as your date range.
There are 1,373 unique visitors who have made a purchase via Checkout in the last 14 days. If you adjust the date range, your Unique Visitors will change accordingly to the total count of visitors tracked during the selected period, while your current audience size will remain the same, since is based on the total number of visitors that are tracked in this audience today.