Most people ride a seat size that is too big for them and this creates several problems. When the seat is too big the rider is often times pushed back in the seat to a weaker part if the horse's back. The longer seat also often puts the center of the rider's balance too far behind the stirrups causing the feet to go out in front and compromising the rider's balance. Another problem is that we often see saddles with a bigger seat needing a longer tree. This results in a lot of horses being sore in a lower back. Often we see riders that have been using 15 and 15½" seat who are very comfortable in a 14" and 14½" vaquero saddle. As a reference Jeff Sanders is ⅝" tall and weights 155 pounds and both of his Vaquero saddles are 14" seat.
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.
Advantages of the center rigging is that the rigging puts even pressure front and back when the cinch is tightened. It does not require flank cinch to keep back end of the saddle from rising up. When doing things like riding downhill or roping cattle not having a flank cinch can also be a benefit when training horse because it is not there to get in a way of your leg signals.
The disadvantages are that it can be harder to keep the saddle in position on some horses. It also requires the rider to stay in much better balance than some of the other rigging styles. It can be especially difficult to rope in if the rider does not ride in a very good balance and does not have a lot of experience sliding rope and handling cattle.