What's the biggest implant I can have without risk of sagging with a breast lift? (Photo)

A full mastopexy breast lift would be an excellent operation for you.  An implant that large at the time of a lift would potentially be a problem with wound healing and blood supply to the nipple.

When getting your breast lift the insertion of an
implant as well, may be advisable to restore superior fullness, which is lost
with aging and after pregnancy despite the rest of the breast being overly
large. You appear to be asking about the maximum size implant that could be used when completing this process, but their are additional risks to placing the 400-500 cc implants that you mention in your question.Over time, gravity will continue to have an effect and the
breast skin will tend to stretch. The degree of #stretching and #sagging varies
between women: generally, women with smaller breasts experience less recurring
sagging. If sagging does occur, further excision of the skin on an outpatient
basis can be used to correct the problem. Heavy and large breasts may lead to
recurrent sagging and may require the removal of a small amount of breast
tissue to achieve an optimal shape and size. One key to a satisfying result is
realistic expectations.

It is very difficult to determine the exact size and shape implant you will require to best match your ideal breast image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your breast will determine, in many cases, the maximal volume per implant profile that you can accommodate. To illustrate; a 100 cc difference may make a significant difference with a narrow base width breast, but much less of a difference if you have a wide chest wall and wide breast “foot print”. Therefore, just because your friend may have a great result with let’s say a 350 cc implant to make her go from a “A” cup to a “C” cup size does not mean that you will have the same result with the same size implant. . Further simply placing implants in a bra to determine the size best for you is not always accurate as the bra often distorts the size, is dependent on the pressure the bra places plus the implant is outside your breast and not under it among other variables. Computer software morphing programs that automatically determine the best implant size can be helpful in some but not all cases (e.g. doesn’t work well in my experience with existing implants, sagging or asymmetric breasts). Using “want to be” photos however are useful if simply provided to the surgeon as I will further explain in the below link:

Larger the implant more is the weight.  Risk of sagging is directly proportional to weight of implants.  Have a breast lift and use an implant that gives desired shape and fullness.  Do not be fixated on size.  Large size will be counterproductive in long haul.

Your question suggests frustration at the lack of uniformity among plastic surgeons and a lack of understanding of what you are asking for. You specugucally asked how large of an implant can be put in your (already) sagging breasts which would NOT risk further sagging. What you are asking for defies gravity and the laws of physics and aging. If your current I enhanced breasts sagged already why would they magically not sag again if any implant was added to them? Of course they would. Since current implants are filled with either saline or silicone gel NOT with lighter than air helium , even the smallest breast implant would add weight to the breast and pull it down. The larger the implant the greater the weight and the faster the pull and sagging. If you cannot accept this fact do not have the surgery. That us why some surgeons would rather not end sabotaging their breast lifts with large implants. If you wish to proceed you have three options:- breast lift alone - breast lift with immediate or delayed augmentation  delayed augmentation with yiur own fat. The most accurate way to control your breast size and shape would be to do the lift first and do the augmentation with implant or fat later. But it is up to you one what makes most sense to you. Good luckpeter Aldea , MD