There is no strict classification in the size between small and large Klins.
Ogama should be able to burn the same quality of ceramics as Old Bizen(of Momoyama era).
From this point, at least 20m in length(x 2m width x 2m height) should be necessary.
The size of old Ogama was 50m in length(x 5m width x 2-3m height).
Master Togaku and his team own three large klins(20, 50, 85m).