As the title says, how long? I want to put a loco + tender length Cassette at the both ends of each track of the traverser so I can lift off loco, put an empty cassette in its place, hand shunt the coaches to the end, remove cassette from other end of the train and place the turned loco ready to take the train out, or to be replaced by another loco. The longest  I have at the moment is just over 6", so I'm thinking 7" should be enough to cope with anything that might "visit" like an A4 or Duchess.

Given the fragility of some recent steam loco motions the less handling the better. The other fiddle yard has more space and has a Turntable at the inner end and a spare run round track, so I run a loco onto the T/T turn it and run round, same end result.

I'll also probably have one  train length cassette position to cope with "once a day" trains like the Belle, a couple of cross country with other company stock and maybe an excursion or two.