Databases on z/OS and 390 are completely different than on other DB2 platforms. A database on z/OS is just an administrative grouping of tables within one sub-system. Table names must be unique across all databases in a sub-system.
If you want the size of the data in the whole DB2 subsystem, you could do the following:
1. Make sure runstats is run on all tables and indexes (including the catalog). Calculate the size of the tables and indexes based on the number of rows * row length plus overhead.
2. If you use stogroups, run the stospace utility.
3. Look at the size of the underlying VSAM datasets which you should be able identify by a naming convention used for DB2 objects.