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

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.

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