In standard English enormity now relates to size, the hugeness of something. We might say that the enormity of her knowledge was legend in London, or that the enormity of the problem made a solution impossible to find. It has come to mean the same as enormousness.
This is a fairly recent shift in meaning, however. Enormity used to be reserved for very negative criticism and related to the fact that it related to a crime or sin. We might have said that the enormity of the serial killer’s acts defied belief. If we now use the word in its old sense, however, it is likely that very few will understand what we mean. It is better to be understood than to stick to old usage, however superior it may make us feel! Language and meanings change; they always have and will continue to do so.
If you’re as keen on words as we all are, take a look in our bookshop for recommended books: it could give you great ideas for Christmas presents!