You will want to discuss this with your surgeon. Part of the issue is that the larger the implant that is used in someone with a droopy breasts, the less skin that can be removed to lift the breast. It is kind of like a see-saw in that you have shape on one end of the spectrum and...
You are going into surgery being thoughtful about repercussions of size on long term health, which is commendable. I think that you should start with sizers in the office to determine the size with which you would be happy. Then, the weight of the implants for that...