"Importance or reputation gained by ability or achievement: 'an architect of international stature'"

A person of stature in the English language would be, by my consideration, someone who is notable for upholding the generally accepted usage of the English language. An English teacher has been professionally trained in proper English. An editor is tasked with enforcing proper English. Students being professionally (and specifically) trained in the English language would be suitable for being authorities, but they not necessarily of stature.

I didn't think I had worded it ambiguously, but here it is reworded: I don't think anyone who has a reputation gained from achievement in the subject of proper American English would argue that "could care less" is a good substitute for "couldn't care less".