I believe the J-20 slightly longer body is due to the fuel consumption that it needs for the heavier WS-15 engine that's going to be use to provide a more thrust. As far as it's primary roll...we don't know for sure.
True but not exactly. The Chinese military emphasizes supercruise as the foremost attribute of the next generation fighter. In order to achieve that goal with potentially underpowered engines, the J-20 was elongated to minimize drag at supersonic speeds.
I think that the J-20 will be a fighter-interceptor in the vein of the fabled YF-23. While it may never match the Raptor in close-quarter turning, it may not have to (once again I'm sure that it will be no slouch in the maneuvering department). EODAS and wide-off bore missiles, rather than fancy aerobatics, will dictate the outcome of future air-to-air engagements.
Well these looks better than the CGI images I saw but still like the F-22 type tailfins better.