Enormity - enormousness
Although these two words are derived from the same origin (Latin e norm-, ~ 'out of the ordinary'), and in the 17th Century, enormity and enormousness were more or less interchangeable, their meanings are now very different. In academic writing it is regarded as a malapropism to confuse them, so don't do it.
enormity nowadays means basically Extreme or monstrous wickedness or a gross and monstrous offence (OED). It is used about moral or legal deeds that are felt to be particularly wicked or bad. Their size is not the point; it is the shocking way in which they break the rules that is conveyed by enormity.
Don't use enormity to mean 'large size'. That idea, the abstract noun from the adjective enormous, is covered by the word enormousness. This word, however, is felt to be clumsy. Use a synonym like 'vastness' or 'size', or a phrase like 'huge extent'.
(Oddly enough, the adjective enormous itself is not used in the same sense as enormity - although it used to be. Nowadays it only means 'of a very large size, or extent'. OED says This is the only current sense.)