这样的数据库有多大？

I'm trying to figure out how big a certain database would be (it hasn't been created yet). I know how many rows and what the tables will be. Is there a feature in Oracle that will tell me the size of such a theoretical database? Is there a known math formula I can use? I know there is a feature to determine the size of an existing database, but I want to know how big it will be before I create it. I'm trying to figure out how big a certain data