Shorter is better- for once
The max lenght stated by the DEC is 5 meters- as Kobus say.
Interference can be a issue but by careful routing not a huge problem.
The inbuitl shield mesh should prevent that.
I do not think anyone have had any problems.
More than 5 meters can cause a speed problem- but again, not likely.
The max speed of the diagnostic system is actually low compared to other uses.
Haakon