300cc too small? I am almost 5’9 and a size 2/4 with a 34a bra size.

Depending on the individual anatomy and desired results, estimations, measurements and surgical planning should be made for each patient individually. It will be best to consult a board certified plastic surgeon who is experienced in cosmetic breast surgery in person and discuss your goals and options after a physical examination.Hope this helps,Ali Rıza Öreroğlu, MD FEBOPRASEuropean and Turkish Board Certified Aesthetic, Plastic and Reconstructive SurgeonIstanbul, Turkey

Thank you for your question! I would make sure you do a couple "sizings/fittings" to decide what size looks good on you!  Make sure you consult with a board certified plastic surgeon and discuss your desires and results! Best of luck!Dr Dhaval PatelDouble Board CertifiedPlastic SurgeonChicagoHoffman EstatesOakbrookOrland Park

Patients often think in terms of cup size when considering augmentation. Unfortunately, devices are sized in terms of milliliters (cc) of volume. This can lead to some confusion when sizing. Additionally, it is important to remember that cup size itself is not standardized with variations from one manufacturer to another. Unfortunately, as many women can attest their cup size in an industry leader such as VS is not necessarily transferable to another brand.
Another point which is often under appreciated is that of anatomy and starting point. Any implant will add volume to the volume which is already present. The implant is additive. A particular volume will not necessarily confer the same cup size to different patients (often times it will not even confer the same cup size to different breasts in the same individual...remember they are "sisters" not "twins"). 
A general rule of thumb is that 125cc can represent somewhere between 1/2 to a full cup size increase. Smaller volume differentials (25-50cc) are typically less consequential representing a volume change of less than a shot glass. However, I have found these numbers, at least anecdotally, to be of little help. Patients often present with notions/goals which do not correlate with these sorts of sterile volumetric assessments.
When sizing patients, there are a number of useful tools including:
-3D imaging (has the added benefit of offering a volumetric analysis of the pre-operative breast)
-Breast sizers (rice bags)
-Goal photos
I also recommend that patients commit to a particular look rather than a cup size. Once a patient settles on a look that pleases them the overall cup size increase becomes less relevant.The key to obtaining a natural result is to stay within the parameters defined by your BWD. This will ensure that you avoid the dreaded "fake" look. 
Regarding your specific question, it is nearly impossible to predict cup size. For a smaller framed individual, I will typically recommend a high profile device (greater volume for a given BWD).
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

hello thank you for your question and provided information as well..Based on your question, it is difficult to evaluate your case, if you do not have a photo, my recommendations are that you are consulted in person by a certified plastic surgeon who sees and evaluate your case and determines what procedure you apply.

Size is your question, but why is your surgeon NOT recommending submuscular placement? With minimal breast tissue (A cup) and wider chest (34), your height and size suggest minimal subcutaneous fat as well--all leading to visible implant edges, possible ripples (even with silicone), and an unnecessarily higher risk of capsular contracture. 
What profile is your surgeon suggesting? Hopefully not High Profile! Your profile should be chosen to be the widest (at the volume you eventually decide on, but more about that in a moment) that fits your breast base, not "automatically" HP, which too many surgeons (incorrectly) will tell you gives more upper pole fullness. HP implants are narrow, and therefore SHORTER vertically, so this ensures LESS upper pole fullness once they drop into their final position!
In short, I fear you're getting poor (or inadequate) advice with which to make proper decisions. That is why I fear your size choice may be similarly misinformed. I would want to examine your "A" cup breasts and your frame, and then see goal photographs and see what implants you pick with my nurse who has nearly 20 years of experience with implant sizing, not to mention being a breast augmentation patient herself. Proper width 300cc implants will give you a very nice look, but may not give you the size you really want (small "C" or full "C"?). That's why goal photos and sizing are essential to give more information. Listen to your gut as you read other posts--I think your sense is accurate!
Please read my Comprehensive Guide to Breast Augmentation by clicking on the web reference below. If you don't already have the answers to the questions answered in this long informational piece, your own surgeon has not considered enough of your details. Best wishes! Dr. Tholen

300 cc is a modest implant for someone so tall. It may be okay since you are so slim. I would try implants on again to be sure. I would also rethink the above muscle placement. Below the muscle has a lower complication rate and more natural look and feel. I did a lot of above muscle before 1991 and all below the muscle since then, and I feel strongly that below is much better in almost every instance. Discuss with your surgeon. Good luck.

Hello and thank you for your question.  The
best advice you can receive is from an in-person consultation.  The size, profile, and shape of the
implant is based on your desired breast size/shape, your chest wall
measurements, and soft tissue quality.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.   Make sure you specifically look
at before and after pictures of real patients who have had this surgery
performed by your surgeon and evaluate their results.   The most
important aspect is to find a surgeon you are comfortable with. I recommend
that you seek consultation with a qualified board-certified plastic surgeon who
can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D.  FACS
Harvard-trained plastic surgeon

Hello and thank you for your question.  Without an in person exam it is difficult to give you a definitive answer.  Many women spend time deciding on the right size implant before proceeding with surgery. It is important to have this discussion with your board certified plastic surgeon and ensure you are partnering with someone who understands your goals.  Using sizers in your bra can help, so can reviewing before and after photos on women with similar height/weight.  In our office we use the Canfield 3D Vectra to create simulated after photos - finding a resource with this or similar technology may be helpful. Good luck with your decision making process!
Kindly,
Dr. Katerina Gallus

There are so many factors that go in to choosing an implant size for a patient including, height, weight, breast dimensions, chest shape, rib shape, nipple position, skin tone, amount of sagging among others. It is best to place that size implant in a bra with a tight shirt on to evaluate the new look. Imaging software can also be very helpful. Keep in mind that implants placed in a bra in the office for sizing will look slightly smaller when in your body. Also, in some patients implants above the muscle tend to look larger than when placed below the muscle.

The best cosmetic result in any particular breast
augmentation patient depends on a variety of factors, including: Your
individual anatomy, desired outcome, 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 sit
at a preoperative visit will help you and your surgeon choose the optimal
implant.
There are no manufacturers' standards for cup sizing in the bra industry. The
cups of a 34 C and a 38 C  are significantly different. Cup size varies from
manufacturer to manufacturer and even within styles from any particular
manufacturer. There is also no direct correlation between an implant size or
shape and resultant cup size.
Keep in mind that following the advice from a surgeon on this or any other
website who proposes to tell you exactly 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 that your plastic surgeon
be certified 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