36DDD. What is the ideal bra size/ cup for a 4"11 petite woman?

require that minimum weights be removed to qualify and you must see a local plastic surgeon to find out if you meet that (probably).  Reductions only guarantee that you will be smaller and if there is a minimum agreed to, your surgeon  must remove that amount so its remotely possible that you may have more tissue removed than you would have desired.  As for how much to remove, my patients could direct me to be conservative (remove only the minimums), do the procedure how I normally do them, or be aggressive for the smallest outcome possible.

There is no "right answer"; patient preferences play a very large role when it comes to determining final size  achieved with breast reduction surgery.
My best advice: select your plastic surgeon carefully. Make sure that he/she has significant experience achieving the types of outcomes you will be pleased with.
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). 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 “B or C 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