Hi, Bobette:
Especially considering the huge discrepancy in reported size of your AN by two doctors, I think you are wise seeking an opinion from Stanford doctors. Your surgeon is probably correct in reporting 12mm, as he is probably more of an expert than your ENT. But Stanford is the Rolls Royce of CK facilities, and Dr. Steven D. Chang (Stanford neurosurgeon and CK specialist) is perhaps the world's leading CK expert. You can email Dr. Chang at sdchang@stanford.edu and ask him to review your MRI and audiogram. He will give his recommendation for free if you do this by email and phone rather than schedule an office visit.
Dr. Chang told me that the sooner I got CK, the better the chance of preserving what remaining hearing I had left. He told me that only after I asked him how long I could stay in W&W; he never pressured me to make a decision. He is amazing -- as dedicated a doctor can possibly be to his patients. He usually answers emails within a few hours, even on holidays!
CK was extremely successful for me. You are on the right path.
Best wishes,
TW