How to estimate database size growth with larger user account growth

Hi,
I am about to migrate over to aws and setup a load balancing, multiple ec2 wordpress setup and use RDS for the mysql database. My site is a social media site using buddypress that has the goal of 15k new users day one, and will will have huge growth after due to working with a experienced marketing team from a currently owned business. What I want to figure out is as we go from 15k users, to 25k, to 50k, and if lucky enough to get to 100k, to a million user accounts with a large portion active daily, how big of a database will I need? Any user content such as images and videos will be stored on s3.

RDS gives 20GB of storage with AWS free tier, and I want to know how far that will go, and be able to factor in database size if over that to estimate total costs as the userbase increases.

Also I do know that it may be different for every wordpress setup based on alot of variables so even if I cant get a general rough estimate, I want a way to estimate the size based on creating several test user accounts and seeing the file size (eg. where is that stored exactly?)

thanks for any help in advance.