There are many different ways to measure the size of an Oracle database and/or the size of actual space used vs. free space remaining in various Oracle objects.
If you just want a very high-level view as the O/S or storage system sees it, that's easy, just use an O/S command to list the Oracle data files and there sizes.
Beyond that, it gets more complicated. For example, in a brand new system, I could create a new Oracle database and create 10 new tablespaces of 2GB each. From the O/S or storage system, that looks like 20GB used by Oracle (plus whatever space Oracle already used for the SYSTEM tablespace and other default tablespaces) but i haven't put one byte of user or application data in it yet!
Depending on the storgae parameters that were used to create the tablespaces, when I start to create user tables, some of the space in the tablespaces will get used. But the O/S or storage system will not notice the change, unless I fill up a tablespace *AND* it was set to auto-extend. Then, after creati ng some tables, I maybe still haven't loaded in any data yet. As data is loaded, some tables may fill up and extens, and others may have lots of free space yet.
Basically, this is not a simple question in Oracle.
