I didn't know how to describe it, so I stole this from wiki:
"The breastplate should not be fitted in any way that will restrict the horse's movement. Special attention should be paid to the shoulders, chest, and the area between the horse's front legs. In general, a fist should fit between breastplate and the horse's chest, and there should be a hand's width between the wither strap and the withers
. The breast strap should be have some slack, and care should be taken that its buckle doesn't rub the sensitive skin in the area. It should also be adjusted so that the chest straps lie above the point of the shoulder
so that the horse's motion is not restricted."