how to change the distance between the hands into real world distance?

ImHarry

void draw() {
  kinect.update();
  image(kinect.depthImage(), 0, 0);
  text(youAre, 100, 100);
  IntVector userList = new IntVector();
  kinect.getUsers(userList);

  if (userList.size() > 0) {
    int userId = userList.get(0);

    if ( kinect.isTrackingSkeleton(userId)) {
      drawSkeleton(userId);
    }

    float SizeDistance = getJointDistance(userId, SimpleOpenNI.SKEL_HEAD, SimpleOpenNI.SKEL_RIGHT_KNEE);
    //float hdist = PVector.dist(leftHand, rightHand);
    println(SizeDistance);
    if (SizeDistance > 450) {
      youAre = "I Got You'";
    }
    else {
      youAre = "I Find You";
    }
  }
  //c
}