Incredibly large; large beyond comprehension. Nothing can be larger than something that is gigantuan.
A mixture of gigantic and gargantuan.
If I could give you an example of something that is gigantuan it would be comprehensible, and thus not gigantuan.
A confined space where one goes to expel one's rage.
If I ever find the person who keyed my car, they're going to step into my rage cage.