Choosing the right breast implant is a complicated procedure that must balance your body, aging or pregnancy changes, weight loss or gain, the width of your breasts, breast asymmetries and your own personal aesthetic desires. Thus, this is a question that can only be answered through a full and detailed consultation with a plastic surgeon. There is no "perfect" answer, and this decision must be individualized based upon a number of factors.
In your case, it appears that you will also need some form of a breast lift to restore you to a more youthful appearance. This complicates the issue even further. Thus, my recommendation is to have several consultations with plastic surgeons in your area and choose one that seems to best fit your realistic desires. It is possible to keep your breast size if an implant is added, as long as some of your existing breast tissue is removed.