Here’s some photos showing correct shaft height and length. Shaft height and balance of the cart are very important. The height of the shaft should come the point of shoulder of the dog and should not extend more than a few inches past the shoulder. Length of the shafts, while not a safety issue, can affect maneuvering. The longer the shafts, the wider the turning radius. Shorter shafts have a shorter turning radius.
If you do shorten the shafts, make sure you also tighten those traces and move your brakes to the appropriate location. The shafts and cart should be parallel to the ground. Click on photos below for bigger version.