Ah, the enormity of it! Just as the use, or to my mind, misuse, by many people of words such as "disinterested" for "uninterested", "fulsome" for "full", "fortuitous" for "fortunate", and "comprise" for "constitute" has led to the appearance of these disputed meanings in the dictionary, the use of "enormity" for "enormousness" (yes, the word does exist) has led the Oxford Dictionary to accept this meaning as well.
"Enormity" really means "monstrous wickedness", although the second meaning, "great size", is given with the warning that many consider it incorrect.
Etymologically, both the adjective and the noun have evolved from the latin enormis, meaning "out of the norm". At first, they both referred to behaviour out of the norm, as in an enormous sin or the enormity of the act. Then the meanings separated, with the adjective coming to mean "out of the norm" in size, as in an enormous building, while the noun retained its moral meaning. But writers accustomed to the adjective in its new sense of "huge" began to use "enormity" as a corresponding noun instead of the awkward "enormousness". Here is an example.
The enormity of the stakes and the long-term nature of the game make the eventual decisions so important. (Times Colonist, 8 September, 2011).
Fowler, subsequent style guides, and the Oxford dictionary all warn against this usage.
Better to say
The huge importance of the stakes and the long-term nature of the game make the eventual decisions so crucial.
The Guardian (28 November, 2011), on the other hand, in an obituary for Ken Russell, the film maker, uses the word in its proper sense:
It has, of course, to be said that he was capable of almost any enormity in the careless rapture he brought to making his films. He could be dreadfully cruel to his undoubted talent, almost as if he was defying himself, let alone those who supported him.
Here the writer is using the word to refer to Russell's habit of shocking his public by doing something outrageous or "out of the norm".
When a word such as "enormity" has an accepted, long-standing, logical meaning, why not avoid the use of other disputed meanings that leads to confusion?