I'm going to assume two things: you don't want to look fake and you don't want to cause irrepairable harm to your tissues. Is that correct? Once in a while I see a patient that wants to look fake and doesn't care if she stretches and thins her tissues which will cause her severe deformities later, but that is very, very rare. So if you don't want to look fake and damage your tissue, then it is actually not about what you want - it's about your tissues. After all, the implant is going in your breast tissue - not into the wishes in your mind! If a shoe must fit your foot, a heart valve must fit your heart, then so too must a breast implant fit in your breast. If it is too large it will look stuffed, round, and have visible edges. The determination of the ideal implant size for a specific patient has been very well described, and has been published in the top plastic surgery journal. With just several measurements, your surgeon should tell you exactly the best sized implant to fit your tissues. If you go down from there, your upper breast will look empty. If you go up from there, it will look bulgy. It's that simple. One such system is called "High Five" and it was published in Plastic and Reconstructive Surgery by Tebbetts and Adams. This remains the "gold standard" of tissue based planning in that it was the largest such series of patients published to date. I see a lot of patients for revision surgery, and I can tell you that without a doubt allowing patients to dictate implant sizes based upon their wishes for a specific cup size and not what is dictated by their tissues creates lots of problems later!