Get the actual width and height of layer

Point to a layer defined as the target, target.sourceRectAtTime(time, true) will always return the outside bounds of one or more shapes on a shape layer including strokes and layer effects at the current time. Set the second operator to false and only the shape layer object defined by the actual path will be evaluated. Any additional paths or strokes added to a path will not be evaluated. The true or false operator will not ignore layer animators or the Transform>Shape or Transform>Group properties.

I'm not much of a scriptwriter but I do use sourceRectAtTime() all the time.

The best way I know to get any layer real size is to use sourceRectAtTime. The problem is that sourceRectAtTime is only available via expressions. So, a solution is to set an expressions on a text layer sourceText property to retrieve almost any data from the rest of the layers in the comp. Here is one way to do it( you will need to include a JSON  parser to your script):

#include "path/to/your/JSON/file"
/** Get any layer sourceRectAtTime 
 * @function getLayerSourceRectAtTime
 * @param {Layer} layer - The layer
 * @param {Number} time - The time where you want to get the SRT
 * @param {Property} sourceText - The sourceText property of a text layer
 * @param {Boolean} extents - Extents for the sourceRectAtTime expression
 * @returns -Object
 */
function getLayerSourceRectAtTime(layer, time, sourceText, extents) {
  var time = time || layer.inPoint;
  var index = layer.index
  //This is the function that goes on the text layer expression
  //lt => left - top
  //lb => left - bottom
  // ....
  function getSourceRectAtTime(tm, ind, extents) {
    s = thisComp.layer(ind);
    b = s.sourceRectAtTime(tm, extents || false);
    lt = s.toWorld([b.left, b.top], tm);
    lb = s.toWorld([b.left, b.top + b.height], tm);
    rt = s.toWorld([b.left + b.width, b.top], tm);
    rb = s.toWorld([b.left + b.width, b.top + b.height], tm);
    return '{ "lt":[' + lt[0] +',' +lt[1]+ '], "lb":[' + lb[0] +',' +lb[1]+ '],"rt":[' + rb[0] +',' +rb[1]+ '],"rb":[' + rb[0] +',' +rb[1]+ ']}';
  }

  sourceText.expression = getSourceRectAtTime.toString() + '; getSourceRectAtTime(' + time + ', ' + index +', ' + extents +')';

  var worldBounds = sourceText.value.text;

  return JSON.parse(worldBounds);
};


var myRect = getLayerSourceRectAtTime(yourLayer, theTime, yourTextLayerSourceText, theExtens);
var layerHeight = myRect.lb - myRect.lt;