How does a surgeon determine what size of chin implant to put in?

Implantech is a company that many of of use that makes silicone implants and implant "sizers" which allow us to try-in an implant before making the final selection. We place the sizer and evaluate the appearance from all views. The one problem with this company is there description of the size of an implant. Changing from a medium to extra-large may sound significant, but there may be only 3 mm of difference in the actual size. Patients sometimes seem surprised when we place an extra large when in fact, it is only 3 mm larger than the medium. I would prefer that they label their implants in millimeters.

There are temporary sizers the surgeon can place during surgery to see what you look like with different implants in place.  I disagree with anyone who thinks they can "guess" the correct size before surgery as a person's soft tissue and bone structure are unique, and a medium will make a huge difference in some people whereas other people need an extra large, even though before surgery they seem to have a very similar chin.  I do think it makes sense for surgeons to morph a patient's photos before surgery to show what he/she thinks looks good and to agree on this estimate of a final result before surgery, because not everyone's ideal of beauty is the same, ie. your surgeon may have one goal in mind and the patient a different goal, and morphing can help reach a combined goal before the day of surgery.  

When performing chin augmentation with silicone implants, I use a sizer intraoperatively to judge the appropriate projection that is required.  The goal is to improve the chin contour to the level of the lower lip, and no further.  If necessary, the silicone implants can then be customized to your jaw specifically.Additionally, the goal is to ensure meticulous symmetry when placing chin implants.  Be sure to trust your face to a specialist with significant experience, and to discuss these details and consultation.  Best of luck to you.

Aesthetic judgement and experience is used by plastic surgeons in determining size of all implants including chin implants.

Hello from View Plastic Surgery in Korea.
The size of implant is chosen based on how much projection is needed for the chin. Some doctors may provide 3D simulation images for the reference. 
Regards,