The size would be your subscribers - demographics has to be captured by you using survey or subscribe forms. You can enable a simple form that appears after subscribe & set custom questions like name, county, interests etc.
Thank you. To clarify, I mean that the advertiser wants to confirm the number of subscribers I have. Is there a way to share this without a screenshot?
There are some external tools like passionfroot that allow you to connect your Beehiiv account + it creates a storefront. Here is mine: https://www.passionfroot.me/the-sports-stack
It pulls your subscriber data in real time,