Enough people have queried my view that enormity means great wickedness, not mere size, that I've been roused to check all eleven usage and style guides which I own; not a single one approves of using enormity to mean merely enormousness, they all say that enormity should be used only to mean great wickedness or similar. 

So usage such as the enormity of the task facing the rescuers and the government had underestimated the enormity of the problem is likely to cause confusion.  In both cases size would have been a better word.

These are the reference books (all reasonably current) which I consulted:

Fowler's Modern English Usage (Gowers): enormous, enormity the two words have drifted so far apart that the use of either in connexion with the limited sense of the other is inadvisable."

Fowler's Modern English Usage (Burchfield): It is recommended that for the present enormity should not be used in plain contexts where the physical size of an object is the only feature involved.

The Oxford Manual of Style: Extreme wickedness; not immensity.

The Economist Style Guide: It does not mean immensity.

The Financial Times Style Guide: Enormity is something monstrous or wicked; it has nothing to do with size.

The Complete Plain Words: Do not use it in the sense of hugeness.

The Good English Guide:  This misusage [of enormity to mean immensity] is understandable and is now common.

The Chambers Guide to Grammar and Usage: Enormity is sometimes used as a synonym of enormousness, but this is not generally considered correct.

Mind The Gaffe (Trask): Enormity does not mean large size and should not be used to mean this.

Troublesome Words (Bryson):  Enormity does not, as is frequently thought, indicate size, but refers to something that is wicked, monstrous and outrageous.

Lapsing into a comma (Walsh):  Educated writers and speakers reserve enormity for the meaning "great evil or wickedness".  I think the distinction should be observed.  This isn't a case like hopefully, where a purist's misreading of the allegedly loose usage would be relatively benign.

Paul Doherty, 2003.