Chin implants are not soft like breast implants, are made of several different choices of material (solid silicone, e-PTFE, hydroxyapatite, to name a few) and come in different shapes and sizes. They can be used to fill the pre-jowl area (the Mittleman pre-jowl implant) with little anterior augmentation, the entire anterior chin and prejowl area (the Flowers extended anatomic vertical tilt mandibular glove), or just the very front of the chin (the "button" type of implant). Chin implants come in sizes ranging from VVS (very,very small) to XXL, with each implant different from the next by 1mm (or 1/25th of an inch). Custom implants can even be created per your surgeon's direction, and your own bone can be contoured, cut, and reattached using craniofacial techniques.
Sizing your chin implant is something done at the time of consultation with your plastic surgeon, and he or she can help you decide what is best for your anatomy and wishes. If you have orthodontic or bite-mismatch issues, you may be referred to an oral surgeon or orthodontist for correction of these concerns first. I hope this helps!