The above picture shows the front plate as a bit shorter than the back, but they are taken from a one piece torso, so they are proportionate when used together.
Front and back plates are meant to be snug and tied on. The photos below show the plates on a 20" doll with body #1804 stuffed so that the doll is still floppy. The fit may vary depending on the size of the doll, the body size and style, and how tightly the doll is stuffed.
Reborn by Shayla Brooks (Heavenly Footprints Nursery)