In below elbow amputation the length of stump should be
Level of amputation 

For an amputation in a limb, ideal levels were suggested which gave the stump an optimum length to facilitate subsequent prosthetic fitting.
For example :


Above knee amputation : 25-30 cm as measured from the tip of greater trochanter.
Below knee amputation : 15 cm as measured from the tibial tubercle.
Above elbow amputation : 20 cm as measured from the tip of greater tuberosity.
Below elbow amputation : 18 cm as measured from the tip of olecranon.

However, with the recent developments in the fabrication and fitting of prosthesis, it is not necessary to stick to these stump lengths. The viability of the tissue is the main criteria for determining the level of amputation.
10 -15 cm
15 - 20 cm
20 - 25 cm
5 - 10 cm