A Cup Desire to Be a C Cup with 300cc - Will They Look Natural?

In order to achieve the best results possible, I recommend that you consult with a board certified plastic surgeon. Every surgeon utilizes a different technique to determine size of the implants. It is important to me to communicate with the patient prior to surgery to discuss the planned procedure. Intra-operatively, sizers are used to determine the size of the implants to be used to achieve the overall goals of the patient. Providing "goal" photos to the surgeon would be very helpful. Using the sizers, helps me determine which profile and size implant to use. Usually, based on your body type, moderate to moderate plus implants are used. High profile implants may be used to achieve more projection.

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