Silicone Breast Implant Size for Petite Frame?

I'm going to assume two things: you don't want to look fake and you don't want to cause irrepairable harm to your tissues. Is that correct? Once in a while I see a patient that wants to look fake and doesn't care if she stretches and thins her tissues which will cause her severe deformities later, but that is very, very rare.

So if you don't want to look fake and damage your tissue, then it is actually not about what you want - it's about your tissues. After all, the implant is going in your breast tissue - not into the wishes in your mind! If a shoe must fit your foot, a heart valve must fit your heart, then so too must a breast implant fit in your breast. If it is too large it will look stuffed, round, and have visible edges.

The determination of the ideal implant size for a specific patient has been very well described, and has been published in the top plastic surgery journal. With just several measurements, your surgeon should tell you exactly the best sized implant to fit your tissues. If you go down from there, your upper breast will look empty. If you go up from there, it will look bulgy. It's that simple.

One such system is called "High Five" and it was published in Plastic and Reconstructive Surgery by Tebbetts and Adams. This remains the "gold standard" of tissue based planning in that it was the largest such series of patients published to date.

I see a lot of patients for revision surgery, and I can tell you that without a doubt allowing patients to dictate implant sizes based upon their wishes for a specific cup size and not what is dictated by their tissues creates lots of problems later!

The size can vary. It depends on how wide your breast is. Surgeons refer to it as the "Base Diameter" or "Base Width." Only implants that are as wide as your breast or less wide can be used. This will limit the size to some extent.
Implants come in three general profiles. That is moderate, moderate plus or high profile. If you take a moderate profile implant and a high profile implant of the same width, the high profile implant will be able to hold more volume than the moderate profile. This is because it is taller than the moderate profile.
Because it is taller, the high profile has more of a round shape. The moderate profile looks more like an oval. The moderate profile is somewhere in between.
In general, moderate profile implants create less fullness over the superior part of the breast. The moderate PLUS profile has more and the high profile has even more fullness. The more fullness you have over the superior part of the the breast, the more round the breast will look.
Thus, the choice of implant has to do with how big you want to be and what kind of shape you want. If you want a very natural tear drop shape to the breast, you will do better with a moderate profile implant. If you want more of a full round shape throughout, you will do better with a high profile implant.
You should have a very detailed conversation about these options with your surgeon. This will make it more likely you will be happy with your end result.

In the old days, there was only one "profile" implant so you were kind of stuck with a particular size that had a width that matched the width of your breast. This is no longer true.
Now we have several profiles of implant - each of which has a similar width but a different amount of projection. Using a 12 cm breast width, the implants (in this example Allergan's Natrelle ) could be anywhere from 240 cc to 425 cc. Obviously, which is "best" is a matter of your taste. When you visit your plastic surgeon they should be able to help guide you in making this decision.
Good luck!