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,

 Hi, I have performed many facial shaping procedures, including Chin Augmentation with dermal fillers or silastic chin implants, for over 30 years.  An experienced chin implant surgeon should be able to select the proper shape and size of chin implant for you.  There are numerous shapes and sizes of silastic chin implants, some of which are rather old shapes that do not yield the desired "natural" aesthetic results.  The Curvilinear shape is an example that is similar to the old "button" implants (had no wings or lateral tapering elements of the newer EAC design) that created a "pharaoh" shaped chin.  Proper placement and the appropriate approach (placing the incision under the chin and not inside the mouth) are also key to obtaining the best aesthetic result.  You may wish to have this conversation with your surgeon and obtain the implant information. When the chin is weak, this creates an imbalance making the nose appear larger, the mid face top heavy, the lower face looks short, de-emphasizes the lips and allows early formation of a "double chin".  
 Proper placement of a silastic chin implant adds forward projection to the chin thereby creating harmony and balance to the lower face.  Using the same incision, liposuction can be performed to reduce the fat and further shape the neck.  Excess skin, from below the chin, can also be removed through the same incision.  
 I have found that placement of a silastic chin implant, through a small curved incision under the chin (also allows excess skin removal) to be very safe, quick, highly effective and far less invasive than a sliding genioplasty (requires extensive tissue dissection, bone cuts and placement of metal screws and plates to secure the cut segments of bone).  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.  I prefer placement of a  silastic chin implant through a small, curved incision under the chin (submental) instead of through the mouth for the following reasons:
*The submental approach is sterile while the intra-oral approach is not.
*The submental approach requires limited dissection as it is much closer to where the chin implant needs to be placed along the center (front) and lower most sections of the chin on each side (where the wings are placed).  The intra-oral approach by contrast requires dissection all the way down the entire section of the chin in order to reach the same areas for chin implant placement.  This leads to several issues in my humble opinion.  The attachment of the chin muscle to the bone must be cut during the tissue dissection, the sensory nerves (mental) and motor nerves (marginal mandibular nerve) that move the muscles of the lips and mouth) are all in much greater jeopardy of injury because of the added dissection required. In my opinion and experience there is no need to secure a silastic chin implants with sutures and screws when the following conditions are met:
 * Select a silastic chin implant ( I prefer the EAC) that doesn't have too much projection (thickness) as this becomes a problem stabilizing the implant under the periosteum when the implant is too thick.
 * Place the implant through a small curved incision under the chin which allows direct access to the periosteum (under the mentalis muscle) while preserving the muscle attachments.  This prevents upward migration of the implant which can occur when the intra-oral approach is used that severs the muscle attachments.
 * While the center or body of the chin implant should be placed at a perpendicular angle the chin bone in order to provide the proper forward projection to the chin, the "wings" of the implant should be at a lower level and follow the inferior (bottom) edge of the jaw line on either side of the chin.  When properly dissected and placed, this will be below the mental nerve foramen and be just wide enough of a dissection to accommodate the tapering wing.  There isn't enough room in this technique to allow the wings to migrate upward and contact the mental nerve.  In contrast if the dissection, along the sides of the chin (jaw line) are performed too high...this will place the mental nerve in danger and if the side dissection is too wide, the wings will have a space within which to migrate upward.
 * We have all of our chin implant patients avoid touching and feeling their chin implant for 1 month post op.  We also ask that they sleep on a U-shaped airline pillow for the same time.
  Our experience when following the technique described above is that the silastic chin implant does not need to be secured with sutures or screws and does not move.  I have placed silastic chin implants in military, SWAT, LE, professional fighters, as well as actors and many regular people who are active.  In addition, I have had the opportunity on numerous occasions to replace silastic chin implants (placed by others) that had been screwed into the bone.  During the replacement it was evident that the metal screws pushed right through the soft silastic implant as it was tightened down on the hard chin bone.  Which makes perfect sense.  So in the long run, these fixation methods alone are no guarantee that the implant won't move.  Proper placement, proper implant pocket creation and meticulous closure of all the tissues layers is what is required in my humble opinion.
Hope this helps.