Have you got access to the control of the motor? If you have, and it is a stepper motor, the length can be determined from the number of steps. Failing that, how about something like a LVDT (linear variable displacement transducer) - do a Goggle to find out about these devices. These are not cheap - they're normally used in avionics so they will usually have exceptional temperature ranges. Depending on the accuracy required, the electronics can range from trivial to very complex.