What would be the synonym for word massive?

Here are some synonyms for "massive" depending on the context:

Size:

* Huge

* Enormous

* Gigantic

* Immense

* Colossal

* Vast

* Monumental

* Grand

* Tremendous

* Bulky

* Gargantuan

* Mammoth

Weight:

* Heavy

* Weighty

* Substantial

Impact:

* Significant

* Major

* Extensive

* Sweeping

Other:

* Great

* Impressive

* Remarkable

* Extraordinary

The best synonym for "massive" will depend on the specific context and the meaning you want to convey.