Breast width of 12cm. Advised max implant is 270cc. Will this be too small?

270 cc implants are modest size implants.Truthfully, this is a very difficult question
to answer without an in-person examination.That being said, it is probably best to return to your plastic surgeon
to double check the implant size.Best
of luck.

Thank you for the question but, despite your good description of goals, online consultants will not be able to provide you with specific enough advice to be truly helpful. Ultimately, 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, the use of in bra sizers, and computer imaging) 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. I have found that the use of words such as “natural” or "full C 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 (and the attached link, dedicated to breast augmentation surgery concerns) helps. Best wishes.

The best way
to determine the size that fits your personal perception of the perfect breast
is to try on sizers. This way you can see how they fit on you, how they add to
your current volume, and whether they fit your frame. Although the surgeon can
guide you, only you will be able to tell what is the right size. Try on sizers.
HP implants, which are a more modern shape than moderate profile
implants, fit most women better. They have a more appropriate base diameter for
the average women's chest. Mentor HP profile saline implants have base
diameters that range from 8.8 cm (very narrow) to 13.8 cm (moderately wide).
The majority of women in my practice have base diameters in the 11 to 13
cm range. The HP implants fit them well. The moderate implants have
much wider base diameters for the same volume (often in the 14 to 15.6 cm
range). These implants are too wide for most women that I treat. So
when the base diameter is measured and correlated with the diameter of the
implant, it is most often necessary to use HP implants to achieve the volume
the patient is requesting. At the volume selected, my patient's base
diameters are too small for moderate profile implants. Measuring a diameter and
then telling the patient what volume they can have (in high, moderate or
moderate plus implants) in my mind is backwards. The patients should be
allowed to choose the volume. Then the surgeon carefully considers the
base diameter, projection, profile, manufacturer, and fill material that will
achieve the patients goals of size as well as qualitative look (natural,
intermediate, or bold upper pole fullness or projection), all the while taking
into consideration the patient's anatomic features that will affect the
outcome. HP implants can be made to look very natural or very augmented,
depending on where in the range the implants are filled to achieve a particular
volume. At the low end of the fill range, HP implants do not have the upper
pole fullness that most people associate with the fake look. On the other hand,
a fake over-augmented look can be created with a moderate profile implant if it
is overfilled (a common practice for surgeons that commonly use moderate
profile implants). In addition, too wide an implant will put the volume past
the anterior axillary fold (side boob), which in my opinion, looks more fake
than upper pole fullness. So there are many factors that need to be
considered. The experience of the surgeon and the degree to which they
explain the options to you is of utmost importance. The subtlety in the final outcome
is not achieved by which profile implant is used, it is determined by the skill
in the creation of the pocket, the choice of volume, and where in the range the
implant is filled. A 270 cc implant will be very small for your base diameter.  This is a complex decision however, which you need to make with the help of the surgeon.  There is no max.  If that is what you are being told, you need to seek out more opinions.