Formal:
* Eloquacious: This implies not only a large vocabulary, but also fluency and grace in speaking.
* Articulate: This emphasizes clear and precise communication.
* Well-spoken: This is a general term for someone who speaks effectively.
* Learned: This suggests a broad knowledge base that informs their vocabulary.
* Erudite: This is similar to "learned" but carries a stronger connotation of deep academic knowledge.
Informal:
* Wordsmith: This emphasizes skill in using language, often in a creative way.
* Loquacious: This implies someone who speaks frequently and at length, often with a large vocabulary.
* Sesquipedalian: This is a humorous term for someone who uses long and impressive words, but might be seen as overly pretentious.
The best word to use depends on the context and the specific nuance you want to convey.