My bra size is 42F. My breasts are quite saggy and long. What is the smallest bra size I can get down to? (Photo)

Thank you for your question and pictures, yes it is possible to reduce breast size significantly you seem to be a great candidate for this procedure an in person consultation would be best for a detailed exam and evaluation. Always see a board certified PS.

Congratulations on your decision to proceed with breast reduction surgery; it is one of the most patient pleasing operations we perform.
 
Yes, it is possible to reduce the breasts size very significantly. The concern with the amount of tissue removed is related to blood flow to the remaining tissue; if too much tissue is removed in one operation the blood flow to the remaining tissue (including nipple/areola) may be compromised. Part of the tissue that is left in place is called the “pedicle"; this segment of tissue is responsible for delivering the blood supply to the nipple/areola tissue. If the pedicle is made too small (in the effort to reduce the breasts as much as possible) then patient will likely have problems with tissue survival. 
My best suggestion:  seek consultation with board-certified plastic surgeons who can demonstrate significant experience achieving the types of outcomes you will be pleased with. This careful selection of plastic surgeon will be the most important decision you make.
Before undergoing the breast reduction procedure it will be very important to communicate your size goals with your surgeon. Most patients wish to achieve a enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso. I suggest that you do not communicate your goals in terms of achieving a specific cup size. For example, a “C cup” may mean different things to different people and therefore may be a source of miscommunication. In my practice, I ask patients to communicate their goals with the help of goal photographs.
I hope this helps.  Best wishes.