The best way to determine what your breast size should be with respect to your anatomy and give true advice would be a discussion with a board certified plastic surgeon. Not only does proportion depend upon the aesthetic balance of your breasts with your shoulders, waist, hips, and buttocks, but also upon your objectives, lifestyle, soft tissue quality, muscular anatomy, and long-term concerns. A breast reduction also includes a breast lift. There is also a limit to how small you can go due to tissue that needs to be retained for the nipple/areola to maintain a good blood and sensory nerve supply. A PS cannot guarantee that a breast will be a specific cup size after reduction. Very large breasts rarely are reduced by more than two or three "cup" sizes. I never promise a cup size because bras manufacturers do not follow a standard size.
If you are having neck, back and shoulder pain then I recommend The Bellesoma Method. This will reshape your breast tissue creating upper pole fullness without implants, elevate them higher on the chest wall and more medial to increase your cleavage. The weight of the breast is transferred to the underlying pectoralis major muscle resulting in pain relief and long term stability. Vertical scars are avoided, nipple sensation (in 95%) and the ability to breast feed are maintained.Best Wishes,Gary Horndeski, M.D.