How to Design a House
The first is a book by the aptly named "National Kitchen and Bath Association." https://wetransfer.com/downloads/12192e8528d255b63255ed3a38885a6120201128153923/eba9b4d869cb5ad43af58ba9f3d1f1d920201128154013/a4536b (this is a copyrighted book - so maybe not the best to send around) It gives a nice summary of the basic rules of thumb. They are rules, not laws, so not be afraid to bend them, but they are a good starting point for sure.
The second is this blog post: https://www.boardandvellum.com/blog/common-bathroom-floor-plans-rules-of-thumb/ Having designed a fair number of bathrooms, I'd say this covers about 98% of your typical options. Just a really nice concise summary, and better examples than the NKBA book.