I am 5'6 and weigh 210. My breast size is 38DD. What size would I be after removing 500cc for breast reduction?

Congratulations on your decision to proceed with breast reduction surgery; this operation tends to be one of the most patient pleasing operations we perform. Unfortunately, your plastic surgeon ( and online consultants) will not be able to predict, with any degree of certainty, what cup size you will end up wearing after the breast reduction procedure.  In other words, there is no  accurate/specific correlation between amount of tissue removed and resulting cup sizes.
 Most patients  undergoing breast reduction surgery wish to achieve a 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 (determining their goals in regards to breast size after breast reduction surgery) I find the use of photographs of “goal” pictures (and breasts that are too big or too small)  very helpful. I have found that the use of words such as “natural” or “B, C, or D cup” etc. 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.
 
I hope this, and the attached link (dedicated to breast reduction surgery concerns), helps.  Best wishes.