Well, there's two parts, there's the out-of-box part, then the parts you add.

So, start with an app that makes a work object and an assignment, but where you don't add additional properties.  Use a class group so a new schema is automatically created.   Create an assignment.  See how much space is taken up.   Resolve the assignment.  Look at space again and subtract.  Now you have an estimate for your per-assignment usage.

Next, add a page-list, and look at space again and subtract the above, to confirm that the difference correlates well to how much space you expect your page-list to take.

Of course, you're also going to need to have extra space since you don't know if updates to the Pega software will use a bit more space for the out-of-box parts in a new update.    If I were really trying to assess how much space I needed, I'd probably double or triple the values shown in the above experiments, if some hypothetical other department or manager were asking me to spell out "how much space I need".  /Eric

This is a good starting point: https://docs-previous.pega.com/performance-guidance-production-applications-database