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.
So the bytes of logs generated by the specified query would be
(20000-436)/512 = 38.2109375 BYTES