Base width 12.5 cm. what’s the largest implant I can get? (photo)

I appreciate your question
After performing hundreds of breast augmentations, I have found that the size of implant best for you is dictated by your chest wall measurements.  Once we determine that we can choose the profile based on what you want or need to achieve.  If you are seeking a natural look, then the diameter of the implant should be equal to or, more ideally, smaller than the width of your breast. The breast width is a measurement of how wide your breast is at the base, which should be measured at the level of the nipple. Choosing an implant that is smaller in diameter than your breast width will avoid the "side breast" fullness that is often associated with a more artificial appearance. Implants that are wider than this can be more palpable, fall more laterally overtime and also be more palpable. After that we will then choose the implant volume based on the measurements and also looking at any asymmetries you might have. You should choose a board certified plastic surgeon that you trust to help guide you in this decision. Bringing photos as well can help us to finalize, but please understand that these are of other peoples bodies and your results will probably be different.
Silicone will give you a fullness at the top (upper pole fullness). Silicone implants come pre-filled with a silicone gel and are the softest implant available. They feel more natural, which makes them a good option for women with less natural breast tissue; but they require a larger incision. It may be more difficult to realize if this type of implant has ruptured, so it is important to monitor them with annual follow-up visits. Additionally, because this implant contains a more softer silicone (less cross-linked), if this implant should rupture, it may leak but only into the scar capsule formed. However, the newer silicone in these implants may not leak even when the implant shell is ruptured. 
Anatomic gummy bear implants might be a good choice to give you a more natural look. These highly-sought-after, anatomic implants offer a look that more closely resembles the natural silhouette of a breast, and, therefore, are a very attractive option for individuals seeking a natural-looking, aesthetic primary breast augmentation. Additionally, these implants are an especially excellent option for patients undergoing restorative or corrective breast surgery because they provide more stability, shape, and reduced incidence of capsular contracture. Compared to other types of silicone gel implants, the silicone in the cohesive gel implant is more cross-linked; therefore, should the implant shell “rupture,” it maintains its shape and silicone does not leak.
 Implants, indifferent projections. The easiest way to think about it is they called me in a low medium and high projection or profile. Most of the time I use the medium profile which gives people the best in terms of natural look as well as a little bit of projection. High profile implants are for someone that wants  A little bit more of a fuller look. Low profile implants are better for someone that really wants a minimal extremely natural augmentation. Depending upon the anatomic versus round implants there are also subtle differences  in terms of the amount of projection for each profile. 
During your breast augmentation consultation, you should feel the different types of implants available, and try on various implant sizers in front of a mirror to help you to get an idea of how you will look following the surgery. You should also bring pictures of the look you would like to achieve, as well as a favorite top to wear when trying on implant sizers. I also use a 3D imaging system to allow you to try on a variety of shapes and sizes. I prefer this method over adding volume due to implants being under the muscle.
We will also discuss the possibility of needing a lift along with an implant. This will depend on the amount of ptosis (nipple position) as it relates to your breast, inframammary crease and chest wall. Needing a lift or not will also determine where to place the scar for the incision.  In some cases if the the breast and nipple is really ptotic (low) it might be best to stage the procedure by performing a lift first and then doing an implant at a second stage. This will also allow you to have a better understanding of the true size and shape of your breast.
In order to limit pain from breast augmentation and speed the recovery process I offer patients Botox into their muscles as well as a long acting numbing medication called exparel. 
The best way to assess and give true advice would be an in-person exam. Please see a board-certified plastic surgeon that specializes in aesthetic and restorative breast surgery.
best of luck!
Dr Schwartz
Board Certified Plastic Surgeon 
#RealSelf100Surgeon
#RealSelfCORESurgeon

Thank you for the question.  Based on your photograph, I think that you are starting at a good place and should have a very nice outcome with breast augmentation surgery.  There are no hard and fast rules when it comes to diameter of breast implant versus diameter of breast (band width). In other words, these measurements are useful but are only one of the many factors come into play when it comes to selection of appropriate breast implant size/profile.
Ultimately, careful selection of plastic surgeon and careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) will be critical.
 
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. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining 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. For example, I have found that the use of know words such as “natural” or "C or D cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
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, after the use of temporary intraoperative sizers.
I hope this helps. Best wishes for an outcome that you will be very pleased with.