I'm 5'7 What Size Implants Should I Get? I Weigh 155lbs and Currently Am Wearing a 34b?

At size 34, each 100 cc of implant will correspond to 1 cup size change.  If you had 300 cc implants placed you would be a DD and it may be hard to find clothes or bras that fit.  I recommend implant placement behind the muscle and that space limits the volume of implant.  Too large of an implant results in the implant displacement inferiorly and laterally requiring revisions.  I usually recommend the smallest implant possible to patients to meet their desire.  A 200 cc implant would make you a size D.

Best Wishes,

Gary Horndeski, M.D.

 

Selection of breast implants takes many things into consideration and can't be answered just based on your height and weight. The dimensions of your breast, stretchability of your skin, and actual goals are all considered. Make some consultations with a Board Certified Plastic surgeon.

You are asking a very commonly asked question to this forum and unfortunately there is no way to provide a precise answer online.
In order to give you accurate advice, much more information is needed.  Most importantly, physical examination will determine your current situation. A full discussion and communication regarding your desired goals will also be critical in determining which operation and/or breast implant size/type/profile will most likely help achieve your goals.
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or "full C, small D" 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 use  intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the breast implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison. 
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
 I hope this helps.