We had this discussion at the last spring show of the New Zealand Orchid Society in regards to the plant that is in my avatar.Some wanted to put it into the specimen class of Austrailian Dendrobs ,but when it was considered against plants that were that large that they took up 6 feet X 8 feet,It was not.
So using the New Zealand Orchid Societies show schedule.The definition of a specimen is :-

A single plant of unusual size for the genus and exhibiting fine culture and floriferousness.