Looking to get a breast reduction. I’m currently a size 40G. What’s the safest size I can be reduced to?

Thank you for the question and congratulations on your decision to proceed with breast reduction surgery; this operation tends to be one of the most patient pleasing operations we perform.  You will be best off, getting closer to your long-term stable weight (when it comes to safety considerations) prior to proceeding with breast reduction surgery.
  Before undergoing the breast reduction procedure it will be very important to communicate your size goals with your surgeon (who you should choose very carefully). Most patients wish to achieve enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso. 
With the goal of improving communication with my patients, I ask patients to communicate their goals with the help of  computer imaging and/or goal photographs.  Obviously, the outcome associated with the breast surgery will not necessarily match that of goal photographs perfectly, but they do serve as a better communication tool that subjective terms such as "natural", "proportionate", "voluptuous"… Evaluating goal photographs also allows for a plastic surgeon to determine the consistency of the patient's goals and allows for a discussion of realistic expectations as well.  
For example, I have found that the use of words such as “natural” or “C or D cup” or "still have boobs" means different things to different people and therefore prove unhelpful. Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate. 
Once you have communicated your goals carefully, your plastic surgeon will be able to provide you with an estimation of how much tissue will need to be removed (safely), in order to achieve your desired outcome.
Best wishes with your breast reduction procedure; hopefully you will be very pleased with the outcome of the procedure performed.

A breast reduction is often a balance between the patient goals and the limits of anatomy. Your surgeon will know what is necessary with regards to weight resection. Anatomic limits are non-negotiable as a certain amount of tissue must be preserved in order to maintain vascular supply to the nipple areolar complex. Free nipple graft may be discussed in cases where the distance the nipple must be transposed is too great (commonly for sternal notch to nipple distance > 40 cm).
Ultimately, you do play a role in the decision making process. The anatomic restrictions are, on the other hand, more rigid. Technically speaking, the larger breast (and greater the ptosis) the more tissue that will need to be preserved in order to maintain vascular supply to the nipple areolar complex. 
With regards to your specific question, it is difficult to predict the size of safe reduction. However, as stated above, there will be limits based on anatomy. With a very large pre-op cup size (G) it is often unrealistic to expect a smaller "B cup" post op. I generally recommend that very large patients temper their expectations along these lines.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).
Donovan Rosas MD
Board Certified by the American Board of Plastic Surgery
Member: American Society for Aesthetic Plastic Surgery, American Society of Plastic Surgeons
RealSelf Top 100
RealSelf Hall of Fame