Given a high quality barrel, for a LV its length is determined by how much it can weigh to make 10.5 pounds.
It is not uncommon to see a Hunter class barrel contour used on an LV to make weight, since they are smaller in diameter than the typical LV contours. For a HV its the same procedure, you just have more room for heavier/longer barrel.
For either class you weigh the finished rifle less a barrel, subtract that weight form 168 for LV and 216 for HV and you get what the max barrel can weigh. Most smiths subtract one ounce from the max weight to allow for scale error at a match.
FWIW
Steve Kostanich