How can I choose the right cup size before my breast reduction?

Thank you for the question. There is simply too much variability among bra cup manufactures to make it a useful metric when it comes to preoperative communication and/or post operative predictions. 

Before undergoing the breast reduction procedure it will be very important to communicate your size goals with your surgeon (who you should choose very carefully). Most patients wish to achieve enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso.

With the goal of improving communication with my patients I find the use of photographs of “goal” pictures (and breasts that are too big or too small) very helpful. For example, I have found that the use of words such as “natural” or “full C or D cup” means different things to different people and therefore prove unhelpful. Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate.
Once you have communicated your goals carefully, your plastic surgeon will be able to provide you with an estimation of how much tissue will need to be removed, in order to achieve your desired outcome.
Best wishes with your breast reduction procedure; hopefully you will be very pleased with the outcome of the procedure performed.

Regarding breast size after a reduction, there are never any guarantees. For my patients who are undergoing surgery, if they are undecided about size, I have them go to the store and look at bras in the sizes they are contemplating to see the true difference in size between a 38C and a 38D. The problem also in going solely by cup size is that different bra companies are pretty different for the same cup size. You definitely want to make sure you are still proportionate after surgery, but small enough to make a difference in your symptoms if you are having back/neck pain.