The rigging style is generally based on both the kind of withers as well as the kind of bottom line your horse has across the belly. A horse with low withers and more of a round belly is usually better with the ¾ rigging. The ⅝ might also work for that kid of horse. A Center Fire however could have a problem sliding forward on this kind of horse.
A horse with medium withers and medium size belly would probably have no problém with either ¾ or the ⅝ rigging. You might still run a risk of the center fire sliding forward on this kind of horse though A horse with prominent withers and not much belly would be fine with a center fire or any of the other rigging postions.