Your concerns are very real and I will try and answer them in point form:

1) Regarding cup size: It is very difficult to predict the exact cup size after the procedure as bra manufacturers don't have a uniform sizes. When you get a consultation with either me or another plastic surgeon tell him/her the percentage volume reduction you want to achieve and we will tailor the operation to your needs. 2) Will a breast reduction help with the pain?  Yes a breast reduction should help with your upper back and neck pain, provided all other musculoskeletal causes have been ruled out by your GP. 3) Should I wait until I lose my desired 15 kgs first? The risks of breast reduction surgery, such as infection and skin necrosis, are reduced if your BMI is less than 30. Having said that if you find it difficult to lose 15kgs you might just have to go ahead with the operation knowing that the risks are slightly higher. The best advice I have for weight status, is making sure you have a stable weight for atleast 3 months prior to your operation. Optimising your weight is the ideal situation but this can be very difficult especially if you have such heavy breasts making any exercise near impossible. I have many women with BMIs above 30 who have breast reduction surgery and are very happy.