There's a couple of ways of choosing a size for an implant. I think the best way is to actually try on sizers. In our office we have sizers. You can try them on, look in the mirror, and see what look you're most comfortable with.

I wouldn't focus on a cup size per se, I would just focus on the look because who cares what cup size it is as long as you're happy with it? That can be also verified with 3D imaging which will give you a very accurate idea what you look in a 3D image.

Breast Implant Sizing: Concentrate on the Look, Not the Cup Size

During a consultation and the sizing process, Dr. Robert Jenson asks his patients to concentrate more on how the implants feel rather than the cup size.