<prompt> The size of the Fenestra vestibule on an average is3.50 mm long and 2.50 mm wide3.25 mm long and 1.75 mm wide3.75 mm long and 1.50 mm wide4.00 mm long and 2.00 mm wide  <response>: The size of the Fenestra vestibule on an average is 3.25 mm long and 1.75 mm wide. Ref- Dhingra ENT Textbook 6th edition. 