Size of an empty database
First, are you using "database" in the Oracle sense of the term?  Or are you using it in the sense that other database vendors (such as SQL Server or MySQL) use the term?

If you are using "database" in the Oracle sense, that would be the size of the SYSTEM and SYSAUX tablespaces at a minimum and would possibly include the size of the UNDO and TEMP tablespaces.  On a small laptop system, that's probably on the order of 2 - 3 GB but it could be much bigger.  

If you are using "database" in the sense that other database vendors use it, that's what Oracle calls a schema.  An empty schema (a schema with no objects), by definition, consumes 0 bytes of storage.  That's because space is allocated to tablespaces which are independent of schemas-- a single schema can have objects in many different tablespaces, a single tablespace can have objects owned by many different schemas, and a single object (if you're using partitioning) can be spread across multiple tablespaces.