The T139 and X139 models have virtually identical dimensions. The tanks are bigger on the X version, especially the black water tank. My 2003 T139 had a 6 gallon black water tank, and my 2012 X139 has a 25 gallon black tank.
The construction is different, too -- many of the pre-2008 139s were built with wooden frames and aluminum skins, insulated with fiberglass. The newer models have aluminum frames and fiberglass skins, with styrofoam insulation.