It is always better to place the implants under the muscle if all possible. It decreases the risk of implant related complications and offers less implant visibility. 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.