Enormity vs Enormousness
Enormitynoun
the quality of being outrageous
Enormousnessnoun
unusual largeness in size or extent or number
The difference
Enormity means great wickedness or outrage. Enormousness means great size. The enormity of the crime; the enormousness of the building.
How to keep them apart
"The enormity of the task" is the standard misuse — unless the task is monstrous rather than merely large.