Have you taken a look at this documenation: https://community.tealiumiq.com/t5/Customer-Data-Hub/Audience-Sizing/ta-p/13781
Let me know if that's helpful!
AudienceStore is a connector, its setup is described here - https://community.tealiumiq.com/t5/Server-Side-Connectors/AudienceStore-Connector-Setup-Guide/ta-p/1...
So you'd set up the AudienceSizing job as described in the article @EnaBevrnja linked you to, and then link it to an AudienceStore connector set up as per the link above. When you publish and the job starts, you'd then start receiving visitor profiles in the AudienceStore S3 bucket.
To match all visitor profiles in the system, you can use a dummy rule such as;
There is no need to add a new Visitor attribute to match all visitors. In fact, if you do that, you may miss some because the visitor would need to return in order for that new attribute to be set.
If your AudienceSizing job is running for too long, I'd suggest emailing email@example.com for them to check on it.
Are you sure you set up the rule with the OR as per my screenshot?
Lifetime event count is assigned, OR Lifetime event count is not assigned?
If you did, then you need to reach out to firstname.lastname@example.org so that they can reproduce this and help you.