When you ask for the h value in that way you are asking the template dom. The value reported will be for when it was loaded. You need to ask the Layout Dom for the same value. It will have the updated information. You can use code like this:
var oH = xfa.layout.h(txtDescription, "in");
this.h = oH + "in";
where txtDescription is the name of the field. You can put this code on the Layout Ready event of the field that needs to adjust size.