No higher above the floor than a mother’s own teat (usually low).
Why? When a young lamb stretches out its neck (see below) to nurse, the esophagus elongates and forms a groove to carry milk into the 4th stomach.
If it is not stretched, the milk falls into the first stomach instead of the 4th. But the first stomach was intended for grass or hay. It doesn’t digest milk well. So the lamb’s first stomach enlarges to form a “potbelly” and does not thrive.