Formal:
* Lavatory: This is a general term for a room containing a toilet and sink, but it doesn't necessarily imply size.
* Restroom: This is the most common and neutral term. You could emphasize the size by saying "large restroom" or "spacious restroom".
* Washroom: Similar to "restroom," but it emphasizes the presence of washing facilities.
Informal:
* Bathroom: This is the most common and informal term for a room containing a toilet, sink, and often a bathtub or shower.
* Loo: This is a British informal term for a toilet.
* Head: This is a nautical term for a toilet.
Other Options:
* Powder room: This is a small, elegant restroom typically found in a home.
* Men's room/Ladies' room: These are common terms for public restrooms.
* Facility: This term can be used in a more formal context to refer to a restroom.
Ultimately, the best word will depend on the specific context and the desired level of formality.