I'm 5'6", 130lbs and a 38DD. What cup size would be ideal for my height and weight?

The proper implant size should be determine in person with a surgeon based on your measured dimensions (not weight and height and cup size).  Be sure to discuss your goals and concerns with your board certified plastic surgeon and consider the pros and the cons. Be certain your surgeon is properly certified.  I've attached a link where you can start your search. 

Congratulations on your decision to proceed with breast reduction surgery; it is one of the most patient pleasing operations we perform.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) and computer imaging very helpful.  For example, I have found that the use of words such as “ideal” 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 with your breast reduction procedure; hopefully you will be very pleased with the outcome of the procedure performed.

You would need to be seen in person to determine what volume can be removed.  Final cup size is hard to predict because cup size does not always correspond with volume reduction.