Size:
* Colossal: Extremely large, immense
* Enormous: Huge, vast
* Gigantic: Very large, enormous
* Immense: Extremely large, vast
* Massive: Very large and heavy
* Monumental: Extremely large and impressive
* Prodigious: Extremely large or great
* Voluminous: Of great size or volume
Quantity:
* Abundant: Plentiful, copious
* Copious: Abundant, plentiful
* Extensive: Covering a wide area
* Profuse: Plentiful, abundant
* Substantial: Large in amount or degree
Importance:
* Formidable: Inspiring fear or respect through size or strength
* Grandiose: Impressive in scale or scope
* Magnificent: Impressive in beauty or size
* Monumental: Extremely important or significant
* Notable: Worthy of attention or notice
* Significant: Important or meaningful
Other:
* Ample: More than enough
* Expansive: Wide-ranging, comprehensive
* Imposing: Impressive in size or appearance
* Luminous: Bright, shining
* Overwhelming: Extremely large or powerful
* Vast: Extremely large in area or scope
The best word to use will depend on the specific context and what you want to emphasize.