It's complicated. MOS note 1274720.1 gives an upper limit for the size of the first full backup of a database. But as it notes, this depends on
- the device type used (DISK or SBT) determines whether or not UNUSED BLOCK COMPRESSION is used
- the number of dirty blocks found during the input scan (NULL compression)
- backup compressionAlso
NOTE: The information in this article is a rough estimate and will change since block compression, undo optimization, and other factors may change between backups.But you can calculate a general upper limit as:
Size of the data files - free space in them
For example:
SQL> select sum(bytes)/1024/1024 from v$datafile;
SUM(BYTES)/1024/1024
--------------------
990
SQL> select sum(bytes)/1024/1024 from dba_free_space;
SUM(BYTES)/1024/1024
--------------------
344.25
Giving an upper limit of 990-344.25=645.75 Mb
You can also control the size of a backup set, as described in the docs:
https://docs.oracle.com/en/database/oracle/oracle-database/19/bradv/backing-up-database-advanced.html#GUID-14E97C71-7205-44AD-A711-C5B0053EEA58