What size is right? 5'6" and 127 pounds and wear a 34A. Scared to look too big! What size is right?

Thank you for the question.  
In regards to specific size/profile of breast implant, despite good intentions, online consultants will not be able to provide you with  meaningful specific advice.  Ultimately, in person consultation with well experienced board-certified plastic surgeons will be necessary. Careful measurements, dimensional planning, and careful communication will be key.
Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you're looking for. Ask to see lots of examples of his/her work.  
2.  Again, have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will 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 "big B or small C cup" or "tennis balls" 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.  The use of computer imaging may be very helpful during this communication phase.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery.  The use of temporary intraoperative sizers with the patient in the upright position makes selection of the best breast implant size/profile relatively easy.
I hope this, and the attached link, helps.

Biodimensional planning can help your surgeon determine the right size, shape and position of your breast implant. Sizers or inserts can give you an idea of what it would be like to have fuller breasts, but certainly are not particularly accurate. Even small breast implants can look unnatural if the correct size implant is not used in the ideal position on the chest. One excellent way for patients to communicate their goals is to look at the website of a plastic surgeon you are considering. Find several before images that look like you do to start, with results you find pleasing. The size and shape of the implant may be different from woman to woman based on height, width of chest, etc. But this will help your plastic surgeon to understand your goals. It will also help you to know that you have found a plastic surgeon who shares your aesthetic sensibilities.