What size implants should I choose? I'm 5'4", 140lbs. I'd like somewhat of a natural appearance, mostly to add fullness.

The best cosmetic result in any
particular breast augmentation patient depends on a variety of factors,
including: your individual anatomy, realistic expectations, a thorough
discussion with the plastic surgeon about the options, and an understanding of
the pros and cons of any particular implant choice. Proper sizing is not just
about the number of cc’s. The thickness of your tissue, breast dimensions which
include the width, height, and projection, as well as chest wall width all need
to be considered when choosing an implant. Trying on implant “sizers” of
various shapes and volumes while wearing a tight t-shirt, bra, or bathing suit
at a preoperative visit will help you and your surgeon choose the optimal
implant.

Keep in mind that following the
advice from a surgeon on this or any other website who proposes to tell you
what to do without examining you, physically feeling the tissue, assessing your
desired outcome, taking a full medical history, and discussing the pros and cons
of each operative procedure would not be in your best interest. I would suggest
your  plastic surgeon  becertified by the American Board of Plastic Surgery
and ideally a member of the American Society for Aesthetic Plastic Surgery
(ASAPS) that you trust and are comfortable with. You should discuss your
concerns with that surgeon in person.

Robert Singer, MD FACS

La Jolla, California

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.  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" 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.  Best wishes.