With DCC it's almost but not quite as critical to have the reversing section longer than the longest train using it.
In conventional DC, while the train is in the reversing section, you change the polarity of the
main track (outside the reversing section). This means the polarity of the track leading into the reversing loop and the reversing loop are now opposite. If part of the train hasn't entered the reversing loop yet, every metal wheel that crosses the gap between the 2 sections is going to cause a short circuit.
In DCC, you change the polarity of the
reversing loop while the train is in the reversing loop. Since track polarity does not control direction in DCC, this works fine. But the entry track outside the reversing loop and the reversing loop are still opposite polarities when the change is made. Metal wheels crossing the gap will still cause short circuits.
But if you use an auto-reverser in DCC, the auto-reverser senses the short circuit and changes the polarity of the reversing loop before the circuit breaker can shut things down. In fact, the auto-reverser does not even act until the locomotive causes a short at the
exit end of the reversing loop. If part of the train is still entering the reversing loop when the polarity is swapped, then metal wheels will cause the auto-reverser to reverse again. Then metal wheels at the exit end will again cause a short and cause the auto-reverser to switch the polarity back. The auto-reverser will keep flopping the polarity until there are no more shorts at the entry end. The bad news comes if metal wheels are bridging the gaps at both ends of the reversing loop at the same time. The auto-reverser can't save the day by switching polarities anymore, and the DCC system will shut down with a short circuit indication. And it's possible that the auto-reverser may prematurely release its magic smoke in disgust with your treatment of it.
Solutions: No metal wheels or lighted cars in the section of train that is longer than the reversing loop (and don't ever forget to do this!). Or extend the reversing section to accommodate your longest train. Or run trains that will fit in the reversing loop entirely.
Hope the explanation made sense