What one person might describe as a natural-looking breast augmentation might be described by another as very artificial-looking.
My breast implant rep jokes that I am the biggest user of the smallest implants. In my opinion, breast implants of 450 -500 cc's even on a tall woman would appear to be an obvious breast augmentation.
However, I would never try to convince a woman to go smaller than she wants to be. I would simply suggest that she see a plastic surgeon who shares her aesthetic ideal.
While the size of the implant is critical in ensuring a natural looking result, breast implant position is also critically important. In many patients, the inframammary fold needs to be lowered in order to allow the implant to rest at a level that appears natural relative to the position of the nipple and areola, and in order to prevent the appearance of excessive upper pole fullness.
In profile, the natural-appearing breast is not convex in the upper pole, and an excessively convex and overly full upper pole is a dead giveaway that a breast implant sits below the skin. Likewise, if the inframammary fold is lowered too far, the augmented breast will appear 'bottomed out', with an excessively full lower pole, an empty upper pole, and a nipple/areola that appears to sit too high on the breast - another situation with a distinctly unnatural appearance.
The horizontal position of breast implants also requires a great deal of attention, both in pre-operative planning and in the operating room. Excessive lateral dissection of the implant pockets will result in augmented breasts with an excessively wide space between them in the cleavage area, and the appearance that the breasts are abnormally far apart. Inadequate lateral dissection, on the other hand, will result in an augmentation with an abnormal 'side by side' appearance. As it is lateral projection of the breasts beyond the lateral limit of the chest wall (in frontal view) that, along with the concavity of the waist profile and the convexity of the hip profile, produces the appearance of an 'hourglass figure', careful attention must be paid to ensure that lateral breast projection is not inadequate.
Take your time researching your plastic surgeon and look at many many photographs of women with a starting point like yours and a result that is similar to what you hope to acheive. No two women will have identical results, but by viewing mayn before and after photographs from all positions (front, side and oblique) you will be able to get a sense of whether you and your surgeon share the same idea of what "natural" breast augmentation looks like.