An overly short line will make it easier for the glider to get out of position relative to the tug. An overly long line will result in a sudden lurch by the glider when the line goes taught, and this can result in a ground-loop or worse. My advice would be to tend towards a shorter line initially, and then gradually extend it until you find the optimal length. If, early on, the glider pilot is primed to pull-off the instant he gets out of position (typically too high in the tow, where the risk of a "slingshot" effect, pulling the tail of the tug upwards) then it should be safe to commence with shorter lines to start with.
All the preceding is advice regarding full-size gliding rather than RC, but the same principles apply.