It is R/2 AFAIK.
Lets suppose its a one bit ADC... you now need two take off points from the resistor ladder.
Recall that the top of the resistor ladder is connected to Vcc and bottom to Gnd.
So for two take off points, you effectively need three resistors, say A, B and C so that you get two take off points, ie between A-B and the other between B-C.
Now since the overall quantization is just around two levels in this case, it is customary to have sum of resistors in ladder = 2R.
Hence for symmetry the top and bottom resistors will be R/2, while central one is R.
You can extend the same logic for 2bit, 4 bit etc...
for 2 bit:R/2,R,R,R,R/2
for 4 bit:R/2,R,R,R,R,R,R,R,R/2
Hope it helps...