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.
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