Digital Coax Cable - Does the Length Make a Difference

It might sound counterintuitive, but if you cannot use short cable (<1ft), then you have to use long cable (>1.5m).  Most of the time 1.5m is the minimum so 2m would be safer.

Not sure I understand your answer. Yes, I can use a short cable (.5 meter). Why is 2m safer? 

@ericsch  We want to avoid reflections from the end of the cable.  These reflections appear on characteristic impedance boundaries (changes).  Characteristic impedance is related to geometry and dielectric of the cable and is roughly equal to SQRT(L/C).
We want smooth fast transitions of digital signal.  Reflection can alter them by coming back and adding to the signal.  Typical transition in average CDP is about 25-30ns while the threshold (point of level recognition) lies in about half of that (mid voltage point).  So we don't want reflection back in 30ns/2=15ns. Beginning of the transition starts reflection.  It has to travel to the end of the cable, reflect and come back.  15ns is equivalent to about 3m at 5ns/m signal speed.  It means 1.5m each way.

Now, case for the short cable.  Reflections start appearing when travel time is longer than 1/8 of transition time (cable becomes transmission line).  In our case it would be 30ns/8=3.75ns - equivalent to about 0.75m.  For 25ns transition it would be closer to 0.5m   This distance includes internal wiring of the transport and receiver (DAC).  I would not go further than 1ft cable.  1/8 is a rule of thumb, but normally you would have to know the fastest transition point, calculate frequency, then the wavelength and take 1/10 of it.