I discovered a new theory on facial beauty called the Circles of Prominence.  Based on this theory, the bottom of your chin should be about three iris widths in height from the center of your lower lip to the bottom of the chin.  This is ideal in a female.

But for males, this distance can be larger. Males are distinguished significantly by the appearance of their jaws or lower third of the face.

Also the prominence at the bottom of the chin should be about 3 iris widths in width but in men can be larger.  The larger the chin the more masculine.  If you were to draw a circle 3 iris widths centered at the center of the lower lip, you will delineate the area that should stand out in your mouth area from the rest of the face.

Depending on these parameters, I would decide if you would need a chin implant or a genioplasty.  Generally if the vertical height that you are wishing to increase in your chin is greater than 3-4 mm you will be better off with a genioplasty instead of a chin implant.

Your jaw can also be augmented with a lateral jaw implant depending on what you look like and this can be done inside your mouth without external incisions.  This lateral jaw implant can be used to widen the lower jaw area.