I'm 5'1 and 118 lbs. What would you recommend; 550cc or 400cc?

Hello! Thank you for your question. In general, implant size does not correlate with bra cup size. The cup size itself will vary from manufacturer to manufacturer as well as who is doing the actual measurements. Thus, cup size is never a reliable indicator for your breast size. I typically encourage my patients not to communicate her desires in cup size but more on the actual look and appearance. Good communication between you and your surgeon of your expectations is warranted - choosing your surgeon wisely is the first step. Discussion of your wishes and having an honest and open dialog of your procedure is mandatory (e.g, implant type, incision, placement, lift, etc). I have found that photographs brought by the patient is helpful to get a visualization of the appearance you wish for in terms of size, shape, fullness, etc. In addition, your surgeon's pre and postoperative photographs should demonstrate a realistic goal for you. Once this has been accomplished, allow your surgeon to utilize his/her best medical judgment during the procedure to finesse the best possible result for you after preoperative biodimensional planning and fitting the right implant for your breast width. Hope me that this helps! Best wishes for a wonderful result!

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 are 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 "C or D cup" or "big" 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, viewing the patient's breasts in the upright and supine position, are very helpful during the breast implant selection process.
I hope this, and the attached link, helps.