When i measured missiles in J-20's bay, i got more or less the same dimensions to pl-12. So if it is longer, it's longer by a percent or so, as much as measurement error can be. But it could also be that missiles have identical body dimensions. Keep in mind, pl-12, as sd-10, is 3934mm long (not 3,85m as sometimes is written on the internet)
if what we saw in j20 (and lately on j16 and j10) is indeed pl-15, then its classification as medium/long range missile must stem from the technological edge, but not from doctrinal or weight class edge. And those classifications change over time. What was regarded as medium range missile in 1960s is a short range missile today. (1960s sparrows having similar range to today's sidewinders)