There is no set length for passing sidings. What you need to do is develop your operating plan (what type of trains, how many cars, frequency of trains in an operating session, etc), come up with an average train length, and base your siding length from that. Of course it goes without saying that layout size restrictions need to be factored in as well.
Having worked briefly as a train dispatcher we had to be aware of siding length vs. train length when planning a meet or letting a higher priority train overtake a lower one so that we would use the right length siding. Sometimes that would mean that the train taking the siding might have to sit for a while, but that's life on the railroad.
Of course on a layout that is just one more aspect of operation to make things more realistic.
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Good Luck!
Joe P :D
Long Live The EL