How can I in Oracle discover the average number of rows affected per transaction? Our Oracle server is a big cluster of three very large individual instances. I want to try to understand the average number of rows per transaction. How can I go about doing this?
Not exactly an answer to your question but amount of changes generated by the session can be found using such query.

select st.inst_id, trunc(st.value/(1024*1024)) as MB, ss.sid, ss.username,
  ss.osuser, ss.machine, ss.program, ss.logon_time
from gv$sesstat st, v$statname sn, gv$session ss
where st.statistic# = sn.STATISTIC# and sn.name = 'redo size'
  and st.inst_id = ss.inst_id and st.sid = ss.sid and ss.type != 'BACKGROUND'
order by 2 desc;


Number of rows changed correlates with amount of redo generated.