Breastplates (also known as breast collars) are essential items of tack for a horse whose body shape encourages saddles to slip and slide. But an improperly fitted breastplate isn't likely to help and may even make matters worst.
To check the fit of a breastplate, first see if it's sitting in the correct position across the horse's chest. Single-strap models cross above the point of the shoulder, but they do not rest so high that a horse had trouble lowering or stretching his neck. On a Y-shaped breastplate, the center of the Y will sit slightly above the center point of the chest.
Next, slip your hand between the straps and the horse at various locations. Ideally, there will be enough room for your fist, but not much more. A properly fitted elastic breastplate will be a bit more snug but will not have any "stretch" when the horse is standing still.
If the breastplate has a strap that crosses over the withers or neck, pull straight up on it—it should easily rise three or four inches above the horse.
Finally, check any straps that lead to the girth. Ideally, these will hang two or three inches below the skin, but not much more.