Oracle Dictionary is a wonderful Pool of many hidden resorces. In the Oracle System Statistic Table V$SYSSTAT you can find the Number of blocks written to the Redo. This can be found in the Attribute STATISTIC# = 71. If you measure this value before and after the transaction, you can approximately find out how much Redo-Log the transaction generates. Of course this is approximately, because there may exist other transactions at the same time.
SQL> select * from v$sysstat where statistic#=71;
STATISTIC# NAME
---------- ----------------------------------------
71 physical writes direct
Example
SQL> select value from v$sysstat where statistic#=71;
VALUE
---------
436
SQL> create table myhelp as select * from system.help;
Table created.
SQL> select value from v$sysstat where statistic#=71;
VALUE
---------
20000
So the bytes of logs generated by the specified query would be
(20000-436)/512 = 38.2109375 BYTES
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