I think it depends how the rest of your body looks.
If your are thin, little muscles, a femalish face, anything bigger than AA will push you past androginy, but for someone with a bigger frame and a more male looking face (but still ambiguous), an A cup would be fine.
Physical androgyny is about dosing various attributes to project something that will confuse the other person's normal gender recognition. If a person hesitates to even call you sir or mam, you've done your job right.