General Size:
* Enormous
* Immense
* Vast
* Gigantic
* Tremendous
* Massive
* Colossal
* Mammoth
* Monumental
* Prodigious
* Gargantuan
Extreme Size:
* Humongous
* Whopping
* Outsized
* Overwhelming
* Unfathomable
Figurative Size:
* Unparalleled
* Unprecedented
* Titanic
* Brobdingnagian
* Herculean
For a more specific synonym, please provide context!
For example, are you talking about the size of a building, a crowd, or a problem?