In order to see a visible change in size, a minimum of 60cc (2 ounces) must be considered for the slightest change. I generally have found (in thousands of breast augmentations patients over 25 years) that it takes 250cc to equal about one cup size, so going down by that amount would leave a very small, narrow implant sliding around in a too-large pocket. So compromise must be considered between your "too large" present result, and small(er) implants.
Going down about 100cc, and using moderate profile implants (to get the widest base diameter for the smaller size you and your surgeon choose), will give you the best compromise between the smaller size you wish and too small implants sliding around in your larger already-created pockets. Smaller decrease than this will not be an appreciable difference, and even 100cc is less than a half cup size decrease. But any smaller and you end up with what I describe as a "baseball in a bowler hat" analogy. You will just have a different sort of dissatisfaction.
Discuss this carefully with your surgeon. Also, remember bra sizes are so variable that the letter on any particular bra's label is NOT your size--it's just the bra you have selected. The point here is that you don't want to be a bra size, you want a certain breast size. Then you can buy whatever bra fits well and looks nice, regardless of the letter on the label! Compare the breast implant sizes with the breast appearance on our website bu clicking on the web reference link below--each of these cases include accurate height, weight, and implant sizes--the cup sizes are determined by the patient and are "all over the board." Best wishes! Dr. Tholen