Are 350cc moderate plus silicone too large for my 5'4" 125 frame?

350 mL would probably do quite well. There good sizing systems or you can try out this volume prior to having the operation.

An exam and measurements would give the best info to determine the appropriate sized implant.  Based on my experience this size implant will produce a nice result. 

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 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 D and a 38 D are significantly different. Cup size varies from
manufacturer to manufacturer and even within styles from any particular
manufacturer.

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 you find a plastic surgeon
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