General synonyms:
* Giant
* Massive structure
* Colossus
* Behemoth
* Towering structure
* Immense block
* Solid mass
* Single entity
More specific synonyms:
* Megalith (for a large, ancient stone structure)
* Statue (if it's a large carved figure)
* Monument (if it's a significant structure)
* Blockbuster (if referring to a successful movie or book)
* Powerhouse (if referring to a powerful company or organization)
Figurative synonyms:
* Titan
* Juggernaut
* Leviathan
The best synonym will depend on the specific meaning you want to convey.