It is a common mistake to use "enormity" to simply mean something very large. Indeed, although the text of the defintion below is the correct definition, one of the examples seems to imply "size" instead of "monstrousness".

Enormity does mean immoderately large; however, it has come to mean (something) immoderately atrocious, so awful that it defies comprehension.

If you merely wish to describe something immoderately large, use "enormousness" or "immensity" instead.

An example of the distinction immediately springs to mind; out of respect, and in order to not sound like a deranged schoolmarm, I will leave it to your imagination.

E*nor"mi*ty (?), n.; pl. Enormities (#). [L. enormitas, fr. enormis enormous: cf. F. 'enormit'e. See Enormous.]

1.

The state or quality of exceeding a measure or rule, or of being immoderate, monstrous, or outrageous.

The enormity of his learned acquisitions. De Quincey.

2.

That which is enormous; especially, an exceeding offense against order, right, or decency; an atrocious crime; flagitious villainy; an atrocity.

These clamorous enormities which are grown too big and strong for law or shame. South.

 

© Webster 1913.