in the installation guide,
3.3 Oracle Database XE User Data Limitations
The maximum amount of user data in an Oracle Database XE database cannot exceed *11 gigabytes.* If the user data grows beyond this limit, then an ORA-12592 error will appear. To use more than 11 gigabytes of user data, upgrade to Oracle Database 11g Standard Edition, Oracle Database 11g Standard Edition One, or Oracle Database 11g Enterprise Edition.
in the 2 Day DBA,
The maximum database size in Oracle Database XE is 5 gigabytes (GB). This includes between 0.5 and 0.9 GB for the data dictionary, internal schemas, and temporary space, which leaves just over 4.0 GB for user data.