Should I go with 600 or 700cc? (Photo)

Hello and thank you for your question.  600-700cc is a large implant and you already have large breasts with some ptosis.  I would recommend a smaller implant with a lift for a more natural appearing and safer result.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.  Make sure you
specifically look at before and after pictures of real patients who have
had this surgery performed by your surgeon and evaluate their results. 
Best wishes and good luck.
Richard G. Reish, M.D. FACS
Harvard-trained plastic surgeon

Patients often think in terms of cup size when considering augmentation. Unfortunately, devices are sized in terms of milliliters (cc) of volume. This can lead to some confusion when sizing. Additionally, it is important to remember that cup size itself is not standardized with variations from one manufacturer to another. Unfortunately, as many women can attest their cup size in an industry leader such as VS is not necessarily transferable to another brand.
Another point which is often under appreciated is that of anatomy and starting point. Any implant will add volume to the volume which is already present. The implant is additive. A particular volume will not necessarily confer the same cup size to different patients (often times it will not even confer the same cup size to different breasts in the same individual...remember they are "sisters" not "twins"). 
A general rule of thumb is that 125cc can represent somewhere between 1/2 to a full cup size increase. Smaller volume differentials (25-50cc) are typically less consequential representing a volume change of less than a shot glass. However, I have found these numbers, at least anecdotally, to be of little help. Patients often present with notions/goals which do not correlate with these sorts of sterile volumetric assessments.
When sizing patients, there are a number of useful tools including:
-3D imaging (has the added benefit of offering a volumetric analysis of the pre-operative breast)
-Breast sizers (rice bags)
-Goal photos
I also recommend that patients commit to a particular look rather than a cup size. Once a patient settles on a look that pleases them the overall cup size increase becomes less relevant.The key to obtaining a natural result is to stay within the parameters defined by your BWD. This will ensure that you avoid the dreaded "fake" look. 
Remember "love the look...not the cup."
Based upon your photos alone, I would guess that 600-700 cc is very large for your frame (actually this would be large on any frame). It is important to remember that over sized implants are accompanied by a host of problems not to mention the added weight. The benefits of a subglandular ("overs") approach in a patient who needs a lift is a better match for positioning (i.e. avoiding the appearance of a high riding implant beneath a sagging breast- which would be the case in a submuscular or "under" approach). If you need a lift, then you will likely be dissatisfied with anything less. I would encourage a lift with a much more conservative implant choice.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).