What size would I be if I'm currently in a 40DD with 600 grams removed?

Breast cup size is measured as a circumference around your nipples vs circumference around your ribcage.  Thus on a small thin woman 600g can be several cup sizes, while on a larger woman 600g could be as little as one cup.  It's all about how big you are.  Take a measuring cup, fill to 600g then imagine that much volume removed from your breasts

At size 40 each 300 grams removed would correspond
to 1 cup size decrease. If you had 600
grams removed you would go down to a 40 C.I recommend a new technique called The Ultimate Breast Reduction™.The breast tissue is reshaped creating upper
pole fullness, elevated higher on the chest wall and more medial to increase
your cleavage.The weight of the breast
is transferred to the underlying muscles resulting in immediate pain relief
without excessive reduction.This
technique avoids the ugly vertical scars of the traditional technique,
maintains nipple sensation and the ability to breast feed.Best Wishes,Gary Horndeski, M.D.

No one can tell you with certainty what size bra you will wear after your breast reduction.  All we can say for sure is that your breasts will be smaller.  Seems crazy, doesn't it?Here is why:  the number of grams removed doesn't necessarily equate one to one with cup size.  AND, I see patients all the time who wear a variety of bra sizes, but there is often very little correlation between the bra size and the actual weight of their breasts.  For example I saw a patient a few days ago who wears a size F bra, but her breasts weighed very little, she had a lot of sagginess (ptosis) and loose skin, and it was mainly the extra skin that filled up her bra.  Today I saw a patient with heavier breasts who wears a DD bra.  I would actually be able to remove a lot more tissue (weight) from the DD patient than the size F patient. I hope that makes sense!It is normal to be a little scared, as you are, before a major operation, such as a breast reduction.  I would definitely discuss your concerns with your plastic surgeon, and just let him know that you don't want to have tiny breasts.  In reality, even with a breast reduction you will likely never have tiny breasts.  Also,remember that the vast majority of breast reduction patients are really happy with their results. Thanks for sharing!

Depending on how tight you wear your bra (the number not the letter), your cup size should change one cup for every 200 grams.  That is a generic rule of thumb and doesn't mean you are correctly fitted into your bra currently.  I would expect you to go down at least 2 and probably 3 cup sizes.  Pics would definitely help as would your height and weight.

 Congratulations on your decision to proceed with breast reduction surgery; breast reduction surgery tends to be one of the most patient pleasing operations we perform.  Well experienced board-certified plastic surgeons will be able to give you a much better “estimate” in person than with online consultation.  You will find that there is no accurate/reliable correlation between amount of tissue removed and specific cup sizes achieved. Remember, as you communicate your goals with your plastic surgeon, that a discussion of cup size can be confusing and/or a source of miscommunication. In my practice, I prefer the use of goal pictures to help with this communication process. Generally speaking, I suggest patients undergoing breast reduction surgery have enough tissue removed to help alleviate/decrease  symptoms they are experiencing,  while preserving enough tissue to feel comfortable about the upper and lower body proportionality.
Best wishes.