There is an DB2 XML Extender manual that you should look at.
XML CLOB columns can be up to 2GB, but if you need to log them (logging is optional on CLOBs) then the maximum is 1GB. If you are using HADR, you must log the CLOB columns.
If you need to use the XML CLOB columns in a stored procedure or XML function, then the limits are much smaller.