While you wait for a more complete answer on the scale of the purge, you might be interested in a past post on the experience of the Red Army's officers:

• How much did the Great Purge effect Soviet ability to fight the Germans during (and after) Barbarossa?
  ^(24 Jun 2013 | 2 comments)
  ^(/u/BeondTheGrave compares and contrasts the capabilities of Soviet officers pre-purge and post-purge.)