General:
* Massive
* Ginormous
* Enormous
* Humongous
* Colossal
* Mega
* Whopping
* Monster
More informal:
* Massive
* Gigantic
* HUGE (all caps for emphasis)
* Big as a house
* A whole lotta
* A ton of
* A million (not literally a million, but a large number)
More specific:
* Fat (can be used for things or people, but can be offensive)
* Hefty (for objects)
* Stupendous (for things that are impressive)
* Outrageous (for something incredibly large or unexpected)
The best word to use will depend on the situation and your intended meaning.