Full, Round Breast Implants for a Petite Woman?

It is very important to communicate your size goals with your surgeon.  In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or “C cup” or "fake looking" means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
I use  intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the press implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
By the way, the most common regret after this operation, is “I wish I was bigger”.
I hope this helps.

Based on your description , you could easily be a B cup or larger. You should consult with a PS and try on sizers to determine your optimal size. I think if you go bigger then a D cup you might be out of proportion. I would recommend placing silicone implants under the muscle via an incision of your choice. Don't try saline, you are too thin and will have a good chance of rippling.

AT 18 you can only receive saline implants. Depending on your breast diameter and soft tissue characteristics, I would choose a moderate plus implant with a width no greater than your BWD.