When creating an Engagement today there is no way to know the size of the population from within the engagement creation screen. The business need for this is to better understand how many users are going to be seeing an engagement. Sometimes a communication is meant for just admins or just for specific Accounts. Having a validation of the size of the audience helps ensure that we have created the audience logic correctly.
The ask would be to have a widget/panel on the engagement audience configuration page, that shows the running expected audience. It may need some filter options within it, such as how many days back to check for active users.
The workaround that we are using now is to open the Audience Explorer and recreate the logic from the engagement. Then it will show us the size of the population using the count of users at the top of the page.
