38DD: If I had 1.3 lb of tissue removed per a breast, about what size would that be post surgery?

 Thank you for the question and congratulations on your decision to proceed with breast reduction surgery; this operation tends to be one of the most patient pleasing operations we perform.
There is no direct correlation between the amount of tissue removed and the ultimate cup size that a patient will wear after breast reduction surgery.  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 proportionate” or “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.

It is impossible to tell you what size you would be based solely on the amount of tissue removed. Bra size and volumes do not always match up.  A guesstimate can be made in person with an exam.

1.3 pounds =0.589670081 Kilograms  I am sure that your experience buying bras has been that you can go to six different stores buy four different bras that fit and when you get home they are all a different size.  It is tricky attempting to demonstrate what your new size will be.   Your plastic surgeon will have a variety of different sized implants that are the same density as your breast tissue. 1.3 pounds=0.589670081 Kilograms = 590cc   Look at a 600cc implant. That is the amount that will be coming off.  In one hand lift your breast, in the other hand lift a 600 cc implant.  Now add enough implants to the implant hand to equal your breast, subtract the 600 cc implant and you are left with about the size you will have left at the end of the procedure.  Now to compound the mystery measure your girth beneath your breasts.  If it is 38 you are wearing about the right size bra, then find the cup size that the remaining implants fit and you have your answer.