{script
LAMBDATALK.DICT["W.unicodeLength"] = function() {
  function countCodePoints(str) {
    var point,
        index,
        width = 0,
        len = 0;
    for (index = 0; index < str.length;) {
      point = str.codePointAt(index);
      width = 0;
      while (point) {
          width += 1;
          point = point >> 8;
      }
      index += Math.round(width/2);
      len += 1;
    }
    return len;
  }
  var args = arguments[0].trim();
  return countCodePoints(args)
};
}

Testing:

{W.length Hello, World!} -> 13

{W.length José}          -> 4
{W.unicodeLength José}   -> 4

{W.length 𝔘𝔫𝔦𝔠𝔬𝔡𝔢}        -> 14
{W.unicodeLength 𝔘𝔫𝔦𝔠𝔬𝔡𝔢} -> 7