How many cup sizes can I go down with breast reduction?

Yes you can go down from a "D" to a "B" size with breast reduction typically if that is your goal.. Some patients want to be relatively small to maximize the potential functional benefits of a breast reduction such as back and shoulder pain. Others ( especially younger patients are more cautious about overdoing the reduction so as to maintain the desired size and shape while still feeling more comfortable. Keep in  mind that an exact bra size after breast reduction is hard to predict but generally a patient who says she wants to be a "B" wants to be fairly small and is going for maximum comfort..

There is a limit to how small you can go with a reduction due to tissue that needs to be kept in order for the nipple/areola to maintain an blood and sensory nerve supply.  If going down from a D cup, usually a small C cup is certainly possible.  A face to face exam is necessary to be more precise.  

You have two components for reach breast: skin and volume.  A breast reduction reduces both of these to create smaller, perkier breasts. You can choose any size breasts you want, but in my experience (over 3,000 reductions) I have found that the best result is when the figure is proportioned.  If you are in proper balance (shoulders-breasts-waist-hips) you will be able to wear clothes comfortably and feel confident about your figure.  Remember that the relief from a breast reduction is not just from being smaller, but also from being perkier.Be sure to see a surgeon who has a lot of experience with this operation.