Google has added the the estimated list size directly in the Audience Manager within Google Ads. Google said the "previously, you were only able to see estimated list sizes for your similar audiences when applying them to campaigns."
You should know that these estimates are available for Google Search audience lists with more than 20,000 users, and for YouTube audience lists with more than 5,000 users. Google said it is "working on expanding coverage in the future."
Here is what it looks like:
Forum discussion at Twitter.
