400cc or 450cc HP Silicone Under the Muscle? (photo)

A 450cc implant will likely make you a D-cup.  A full C-cup would probably be closer to 350-375cc.You are fortunate in that you can look great in all of those sizes, so the choice is yours. Good luck.

You should not be deciding on cc's, that is your surgeon's job. Based on your anatomy, you and your surgeon should come to an agreement about silicone or saline, over the muscle or under the muscle.  You should come to an agreement about size. Modest, "classy but juicy", Big and fake.

 An experienced surgeon will then place progressive sizers in the surgical pockets and serially enlarge the breasts.  The breast enlarges symmetrically at first. Then the lower pole cannot stretch any more and the upper pole starts to enlarge disproportionately. Eventually the upper pole gets full and unnatural or the nipple areola complex starts pointing downward.

 If you want a "classy but juicy" full natural look, we drop back a size at the first sign of deformation.  If you like a zaftig look, increase the size to push the upper pole short of dropping the nipples.

Remember your anatomy will determine what is possible, not your surgeon.  He should be skilled enough to do the best with what you have.If your surgeon is asking you to choose a size in cc's, seek some second opinions.

Best wishes. Knowledge is power.  Luis F. Villar MD FACS

Generally speaking, larger implants lead to more risk of complications.  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