How big should the undo tablespace be if I wish to retain undo data for a week?
It is totally dependent on the rate at which the data is being changed in your database. The Database retains a week worth of statistics about undo generation which can be used to calculate the space required to retain the undo data for a given retention period. Oracle9i Enterprise Manager uses these statistics to provide an advisory which estimates the size of the undo tablespace for different values of undo retention time. Another way to estimate the size of the undo tablespace is to measure the amopunt of archive log data generated in a representative week.