Is 250cc or 275cc better for my frame? I am 5'3 and 100 pounds. (phto)

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.Keep in mind that following the advice from
a surgeon on this or any other website who proposes to tell you what to do
based on limited 2 dimensional photos 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 FACSLa Jolla, California

The difference between 250 and 275 cc is 10%, about 5 tablespoons. Only you can decide which you prefer. Try on gel sizers of a variety of sizes until you feel big enough, go higher until you feel you are too big, then back down until you feel just right. Sizing is a process, and the patient should make the final decision since it will affect her for a long time. Computer imaging can sometimes augment the decision-making process, we use Crisalix. You will likely be fine with either of these sizes, but the final decision should be yours.

Either size is rather modest and will like likely make you a large B or a small C.  If that's what you want either of those would be okay.  Im really conservative size wise, but I would recommend the 275 which will not make you too large at all.  It'll look great!  Good luck!