//Calculate length and width based on the rectangular bounds of the selected layer
var length = layer.bounds[2]-layer.bounds[0]; //Grab the length
var width = layer.bounds[3]-layer.bounds[1]; //Grab the width

//Create a text layer
var textLayer = activeDocument.artLayers.add(); //Make a new layer on the canvas
textLayer.kind = LayerKind.TEXT; //Make that layer a text layer
textLayer.name = "Dimensions"; //Name the layer "Dimensions"
var textReference = textLayer.textItem; //Create a textItem which we'll use to fill the text layer
textReference.contents = "" + length + " * " + width; //Set the contents of that textItem to the length and width