You must remember a d cup in an implant would only actually look a large c in clothes.
Implants always measure bigger than what they look. I was a 34b and had 340cc overs and now a 32e-f but i only look a d-dd in clothes, you wouldnt even guess i was an e-f!.
So def get a 300cc as this would be perfect for you, anything less wud be pointless and you may not even notice mch of a difference. my mate who a size 8 had 340cc from a 32a and hers are so small and nice, just perfect, they look a large c cup! but measure a d XX