How big of a cast for Achilles tendon torn?

Short leg cast. Casting for a torn achilles, if indicated, is a short leg (below knee) cast. This is initially in a gravity equinus (toes downward) position and the angle is gradually increased until the foot is at 90 degrees to the leg.
Above or bellow. Some may use an above the knee cast and some may use a bellow the knee cast depending on various different factors. Have it evaluated and see what the doctor recommends for you.
Long leg cast . Usually the initial cast is a long leg(above knee) cast which sometimes i will cut down to below the knee after about four weeks to avoid further knee stiffness.