General synonyms:
* Comprehensive: Covering or including everything.
* Broad: Wide-ranging; covering a large area.
* Widespread: Occurring or found over a large area.
* Vast: Extremely large in area or amount.
* Ample: More than enough; plentiful.
* Full-scale: Covering all aspects of something.
Synonyms emphasizing length or duration:
* Prolonged: Continuing for a long time.
* Lengthy: Long in duration or extent.
* Durable: Lasting for a long time.
* Sustained: Continuing without interruption.
Synonyms emphasizing depth or detail:
* Thorough: Complete and detailed.
* In-depth: Examining something in detail.
* Comprehensive: Including all aspects.
* Exhaustive: Including everything possible.
Synonyms emphasizing size or scope:
* Massive: Extremely large.
* Immense: Extremely large.
* Gigantic: Extremely large.
* Colossal: Extremely large.
Other related words:
* Detailed: Containing many details.
* Wide-ranging: Covering a broad range of topics or subjects.
* Comprehensive: Including everything.
* Far-reaching: Having a significant impact.
* In-depth: Thorough and detailed.
* Multifaceted: Having many aspects.
The best synonym for "extensive" will depend on the specific context and the nuance you want to convey.