[Волгоградец]

Можно так:

txt_txt._width = 100;
txt_txt.border = true;
txt_txt.multiline = true;
txt_txt.wordWrap = true;
txt_txt.text = "ggg" + "\n" + "gggggggggggggghhhhhhhhhhhhhh" + "\n" + "hhhhhhhhhhhhhhh";

var frmt:TextFormat = txt_txt.getTextFormat();
var metrics:Object = frmt.getTextExtent(txt_txt.text);
txt_txt._height = txt_txt.textHeight + metrics.descent;