I was a 34B, wanted to be a 34C/ small D, have 375cc silicone implants and I'm a 34DD. Want to be smaller, what size?

I'm sorry to hear about your dissatisfaction after breast augmentation surgery. As you can imagine, it is not possible to give you precise advice without direct examination and a full communication of your goals.  Some general thoughts may be helpful to you.
Generally speaking,what breasts look like after removal or down sizing of breast implants will  depend on several factors such as: the quality of skin elasticity (the better the elasticity, the better the skin will bounce back),  the size of the implants used (the larger the implant the more trouble you may have with redundant skin), and the amount of breast tissue present at this time (which may have changed since the time of your breast augmentation). 
Life experience since your breast augmentation procedure, such as pregnancy or weight gain weight loss, will  potentially influence the factors discussed above. If you take these factors into consideration and apply them  to your specific circumstances you may get a good idea of what to expect after the implants are removed or replaced with smaller implants. There is no specific/accurate correlation between cc of breast implants and cup size change. As you know, cup sizes will vary from one bra manufacturer to another and should not be used as a communication tool. In my practice I find the use of goal pictures helpful during the communication process. “Tightening the capsule", otherwise known as capsulorraphy, involves the use of sutures to decrease the breast implant pocket space. Sometimes, it is a useful procedure when patients are having breast implant displacement problems and/or downsizing breast implants.  In my practice, I would ask that you communicate your goals carefully with the help of goal pictures. Then, I use temporary intraoperative sizers (with the patient in the upright position);  this intraoperative sizing technique (dimensional planning) allows for individualized selection of breast implants ( size/profile) to help achieve patient's goals.
You may find the attached link (dedicated to revisionary breast surgery concerns) helpful to you.
Best wishes.

It is very possible to #downsize to smaller implants and also alter the positioning; if possible. However, decreasing is size may also require a Breast #Lift to to #tighten loose skin, better optimize the end result. Any #revision has risks of unhappy outcome or other complications. Also, the right implant for the revision is contingent on shape of body and your desired outcome. Breast revision surgery is more difficult, complicated, and requires more time than primary surgery. There is as much as a 40% chance additional surgery will be needed following the #revision. As far as sizing, there are no promise or guarantee of the specific cup/bra #size and shape you will be after your surgery and results will be rewarding if expectations are realistic. As with any surgical procedure, there are some risks which your doctor will discuss with you during your consultation.

It would help to see photos to determine if a lift is needed in addition to switching out the implants.  What size to go with is a matter of judgment and subjective, just like a D  versus a DD is subjective.  It also depends on how much of your own breast tissue you have.  Make sure you familiarize yourself with the different implant size ranges and carefully consider what size to use before you proceed.