400-450 Cc HP Silicones Undermuscle for Petite Woman. Will Be Enough for the Desired Look? What Bra Size Am I Gonna Be? (photo)

You have a slender body with A cup breasts. A 450 HP implant will get you in the D cup range and should get the look you want.  I agree with prior posts to try the implants on in the office to get a rough idea of the result.  A periareolar incision is recommended due to your poorly defined inframammary fold.  Best wishes

In regards to incision choice, I think a well placed crease incision can be camouflaged very nicely and carry less risk compared to the peri-areolar scar in terms of sensation change.  In order to make an accurate size recommendation, I would need to assess your chest wall and breast mound measurements and characteristics.  Unfortunately, there is not a general rule of thumb or objective criteria to implant selection.

Your plastic surgeon will perform several measurements of your chest wall and breast anatomy and determine a range of implants that both fit your chest wall and reach your desired goals.
The next step is to try on this range of implants in the office with your doctor.   The key to this success is showing your surgeon the body proportion you desire with a bra sizer and allowing your surgeon to guide you to the right implant.   It will be much easier to communicate in implant cc's than cup size when determining the appropriate implant for you.
I wish you a safe recovery and fantastic result.
Dr. Gill

From your desired photos it appears that you want a very well defined rounded look.  If you use a HP 400 to 450 cc this will give you the effect you want.  You just need to be sure that your actual base diameter is not smaller than the width of your desired implant.  Also Vectra 3D imaging can be very useful as well.