The beauty of ServiceNow being a SaaS solution is that we, as consumers of the product, don't need to know this. It is ServiceNow's job to take care of everything for us. However, I'd like to learn why you need to know the size of your database, if you don't mind sharing.

I am pretty sure the only page that we have access to is /stats.do which provides some information about the instance, but not the database size.