Formal:
* Colossal: emphasizing great size or extent
* Vast: suggesting a wide area or scope
* Immense: conveying a sense of overwhelming size
* Gigantic: similar to "colossal," but with a slightly more informal feel
* Monumental: implies impressive size and significance
Informal:
* Huge: a common and simple synonym
* Massive: emphasizes weight or bulk
* Tremendous: conveys a sense of awe or wonder
* Giant: similar to "huge," but with a slightly more whimsical feel
* Hefty: suggests something heavy and substantial
Other options:
* Prodigious: emphasizes impressive size and accomplishment
* Extensive: focuses on the breadth or scope of something
* Unbounded: implies something limitless or without bounds
* Boundless: similar to "unbounded," but more poetic
The best synonym for "enormous" will depend on the specific situation and the effect you want to achieve.