There is no minimum length for the fiber, however please keep in mind that you may require an attenuater inline to the fiber to pad down the levels. If your fiber is too short you may overrun the receiver which can cause short term failure to the receiver and cause errors. By padding the receive signal down should help out. You do not want positive Db levels. Try to get the recieve levels down to the range of -10Db to -20Db. That should work fine...
