Possible outer mandible augmentation
The photo you submitted does not show your entire face, but it appears that you are looking to widen your entire jawline. If so, not only do you want a chin implant to augment to the outer (lateral) aspect of the chin, but you may also wish to consider augmenting farther back on your jaw (mandibular angle implants). Speak to a surgeon familiar with such implants.
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Chin implant shape
You can have various sizes, but the material and shapes do make a difference. Eg Porex is quite square and gives the look you noted; but you can also get the result with either a sliding genioplasty, and combine it with tapering/widening Goretex like implant material that can create any width and shape that 'blends' with the rest of the mandible/lower jaw.
So it is not only the use of a single implant that can help you