Here is the
disposition of the corridors in July 1989, just prior to reunification. Britannia Airways did trooping flights into Gatow with 737-200s.
The crew briefing states that "the corridors extend 10 statute miles either side of the center line". (American spelling since Britannia sourced their plates from Jeppesen.) The briefing also confirms the requirement to call PAN PAN PAN to Berlin Center for weather deviations outside the corridors. Another requirement was to have the Interception Procedures readily to hand. The statute (rather than nautical) miles spec probably derives from the terms of the 1945 Potsdam Agreement.