Magic number in optimal redo size query in Oracle 9i
First sentence in the blog post:


  The Oracle Database Performance Tuning Guide recommends switching logs
  at most once every twenty minutes.


Official source:

Sizing Redo Log Files


  It is not possible to provide a specific size recommendation for redo
  log files, but redo log files in the range of a hundred megabytes to a
  few gigabytes are considered reasonable. Size your online redo log
  files according to the amount of redo your system generates. A rough
  guide is to switch logs at most once every twenty minutes.