My experience has shown me that a real treat is when you can see under the snout to make out the bottom dark nebula transition to some light nebula under the snout. This generally takes quite a bit of mirror but when that can be detected with certainty you know you have a fine view.

I would agree with Uwe that this happens in the 20 inch plus range and is definite in the 33" and larger scope given a good sky.

The more trained the eyes are in averted observation of dark nebula with low contrast the better the perception will be of this very difficult object for any given scope.